Date: Mon, 11 Nov 2019 11:44:07 +0100 From: Hans Petter Selasky <hps@selasky.org> To: sgk@troutmask.apl.washington.edu, Mark Johnston <markj@freebsd.org> Cc: freebsd-current@freebsd.org, freebsd-x11@freebsd.org Subject: Re: unkillable process consuming 100% cpu Message-ID: <3e11232b-e2a6-9169-adb0-da0e94523b39@selasky.org> In-Reply-To: <dc2aa04a-6f7b-e369-57c8-b9555df4dd15@selasky.org> References: <20191107202919.GA4565@troutmask.apl.washington.edu> <20191107203223.GF16978@raichu> <20191108220935.GA856@troutmask.apl.washington.edu> <6a4e5993-623a-ebaa-8180-e11c7d48e706@selasky.org> <dc2aa04a-6f7b-e369-57c8-b9555df4dd15@selasky.org>
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Seems like we can optimise away one more write memory barrier.
If you are building from ports, simply:
cd work/kms-drm*
cat seqlock.diff | patch -p1
--HPS
--------------548D5A39F27368510CB30DAE
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diff --git a/linuxkpi/gplv2/include/linux/reservation.h b/linuxkpi/gplv2/include/linux/reservation.h
index b975f792c..0ce922a0e 100644
--- a/linuxkpi/gplv2/include/linux/reservation.h
+++ b/linuxkpi/gplv2/include/linux/reservation.h
@@ -94,7 +94,7 @@ reservation_object_init(struct reservation_object *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- __seqcount_init(&obj->seq, reservation_seqcount_string, &reservation_seqcount_class);
+ seqcount_init(&obj->seq);
RCU_INIT_POINTER(obj->fence, NULL);
RCU_INIT_POINTER(obj->fence_excl, NULL);
obj->staged = NULL;
diff --git a/linuxkpi/gplv2/include/linux/seqlock.h b/linuxkpi/gplv2/include/linux/seqlock.h
index e86351810..115ad5e68 100644
--- a/linuxkpi/gplv2/include/linux/seqlock.h
+++ b/linuxkpi/gplv2/include/linux/seqlock.h
@@ -1,410 +1,148 @@
#ifndef __LINUX_SEQLOCK_H
-#define __LINUX_SEQLOCK_H
-/*
- * Reader/writer consistent mechanism without starving writers. This type of
- * lock for data where the reader wants a consistent set of information
- * and is willing to retry if the information changes. There are two types
- * of readers:
- * 1. Sequence readers which never block a writer but they may have to retry
- * if a writer is in progress by detecting change in sequence number.
- * Writers do not wait for a sequence reader.
- * 2. Locking readers which will wait if a writer or another locking reader
- * is in progress. A locking reader in progress will also block a writer
- * from going forward. Unlike the regular rwlock, the read lock here is
- * exclusive so that only one locking reader can get it.
- *
- * This is not as cache friendly as brlock. Also, this may not work well
- * for data that contains pointers, because any writer could
- * invalidate a pointer that a reader was following.
- *
- * Expected non-blocking reader usage:
- * do {
- * seq = read_seqbegin(&foo);
- * ...
- * } while (read_seqretry(&foo, seq));
- *
- *
- * On non-SMP the spin locks disappear but the writer still needs
- * to increment the sequence variables because an interrupt routine could
- * change the state of the data.
- *
- * Based on x86_64 vsyscall gettimeofday
- * by Keith Owens and Andrea Arcangeli
- */
+#define __LINUX_SEQLOCK_H
#include <linux/spinlock.h>
#include <linux/preempt.h>
-#include <linux/lockdep.h>
#include <linux/compiler.h>
#include <asm/processor.h>
+#include <linux/lockdep.h>
#include <linux/atomic.h>
-
-/*
- * Version using sequence counter only.
- * This can be used when code has its own mutex protecting the
- * updating starting before the write_seqcountbeqin() and ending
- * after the write_seqcount_end().
- */
typedef struct seqcount {
- unsigned sequence;
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- struct lockdep_map dep_map;
-#endif
+ volatile unsigned sequence;
} seqcount_t;
-
-#define lockdep_init_map(a, b, c, d)
-
-static inline void __seqcount_init(seqcount_t *s, const char *name,
- struct lock_class_key *key)
+static inline void
+seqcount_init(seqcount_t *s)
{
- /*
- * Make sure we are not reinitializing a held lock:
- */
- lockdep_init_map(&s->dep_map, name, key, 0);
s->sequence = 0;
}
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-# define SEQCOUNT_DEP_MAP_INIT(lockname) \
- .dep_map = { .name = #lockname } \
-
-# define seqcount_init(s) \
- do { \
- static struct lock_class_key __key; \
- __seqcount_init((s), #s, &__key); \
- } while (0)
+#define __seqcount_init(a,b,c) \
+ seqcount_init(a)
-static inline void seqcount_lockdep_reader_access(seqcount_t *s)
-{
- seqcount_t *l = (seqcount_t *)s;
- unsigned long flags;
-
- local_irq_save(flags);
- seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
- seqcount_release(&l->dep_map, 1, _RET_IP_);
- local_irq_restore(flags);
+#define SEQCNT_ZERO(lockname) { \
+ .sequence = 0 \
}
-#else
-# define SEQCOUNT_DEP_MAP_INIT(lockname)
-# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
-# define seqcount_lockdep_reader_access(x)
-#endif
-
-#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
-
-
-/**
- * __read_seqcount_begin - begin a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
- * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
- * provided before actually loading any of the variables that are to be
- * protected in this critical section.
- *
- * Use carefully, only in critical code, and comment how the barrier is
- * provided.
- */
-static inline unsigned __read_seqcount_begin(seqcount_t *s)
+static inline unsigned
+__read_seqcount_begin(seqcount_t *s)
{
unsigned ret;
repeat:
- ret = READ_ONCE(s->sequence);
+ ret = s->sequence;
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;
}
- return ret;
+ return (ret);
}
-/**
- * raw_read_seqcount - Read the raw seqcount
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * raw_read_seqcount opens a read critical section of the given
- * seqcount without any lockdep checking and without checking or
- * masking the LSB. Calling code is responsible for handling that.
- */
-static inline unsigned raw_read_seqcount(seqcount_t *s)
+static inline unsigned
+raw_read_seqcount(seqcount_t *s)
{
- unsigned ret = READ_ONCE(s->sequence);
+ unsigned ret = s->sequence;
+
smp_rmb();
- return ret;
+ return (ret);
}
-/**
- * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * raw_read_seqcount_begin opens a read critical section of the given
- * seqcount, but without any lockdep checking. Validity of the critical
- * section is tested by checking read_seqcount_retry function.
- */
-static inline unsigned raw_read_seqcount_begin(seqcount_t *s)
+static inline unsigned
+raw_read_seqcount_begin(seqcount_t *s)
{
unsigned ret = __read_seqcount_begin(s);
+
smp_rmb();
- return ret;
-}
-
-/**
- * read_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * read_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
- */
-static inline unsigned read_seqcount_begin(seqcount_t *s)
-{
- seqcount_lockdep_reader_access(s);
- return raw_read_seqcount_begin(s);
-}
-
-/**
- * raw_seqcount_begin - begin a seq-read critical section
- * @s: pointer to seqcount_t
- * Returns: count to be passed to read_seqcount_retry
- *
- * raw_seqcount_begin opens a read critical section of the given seqcount.
- * Validity of the critical section is tested by checking read_seqcount_retry
- * function.
- *
- * Unlike read_seqcount_begin(), this function will not wait for the count
- * to stabilize. If a writer is active when we begin, we will fail the
- * read_seqcount_retry() instead of stabilizing at the beginning of the
- * critical section.
- */
-static inline unsigned raw_seqcount_begin(seqcount_t *s)
-{
- unsigned ret = READ_ONCE(s->sequence);
- smp_rmb();
- return ret & ~1;
-}
-
-/**
- * __read_seqcount_retry - end a seq-read critical section (without barrier)
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
- *
- * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
- * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
- * provided before actually loading any of the variables that are to be
- * protected in this critical section.
- *
- * Use carefully, only in critical code, and comment how the barrier is
- * provided.
- */
-static inline int __read_seqcount_retry(seqcount_t *s, unsigned start)
-{
- return unlikely(s->sequence != start);
-}
-
-/**
- * read_seqcount_retry - end a seq-read critical section
- * @s: pointer to seqcount_t
- * @start: count, from read_seqcount_begin
- * Returns: 1 if retry is required, else 0
- *
- * read_seqcount_retry closes a read critical section of the given seqcount.
- * If the critical section was invalid, it must be ignored (and typically
- * retried).
- */
-static inline int read_seqcount_retry(seqcount_t *s, unsigned start)
+ return (ret);
+}
+
+static inline
+unsigned
+read_seqcount_begin(seqcount_t *s)
{
+ return (raw_read_seqcount_begin(s));
+}
+
+static inline unsigned
+raw_seqcount_begin(seqcount_t *s)
+{
+ unsigned ret = s->sequence;
+
smp_rmb();
- return __read_seqcount_retry(s, start);
+ return (ret & ~1);
}
+static inline int
+__read_seqcount_retry(seqcount_t *s, unsigned start)
+{
+ return (unlikely(s->sequence != start));
+}
+static inline int
+read_seqcount_retry(seqcount_t *s, unsigned start)
+{
+ smp_rmb();
+ return (__read_seqcount_retry(s, start));
+}
+
+static inline void
+raw_write_seqcount_begin(seqcount_t *s)
+{
+ atomic_add_int(&s->sequence, 1);
+}
-static inline void raw_write_seqcount_begin(seqcount_t *s)
+static inline void
+raw_write_seqcount_end(seqcount_t *s)
{
- s->sequence++;
smp_wmb();
+ atomic_add_int(&s->sequence, 1);
}
-static inline void raw_write_seqcount_end(seqcount_t *s)
+static inline void
+raw_write_seqcount_barrier(seqcount_t *s)
{
+ atomic_add_int(&s->sequence, 1);
smp_wmb();
- s->sequence++;
-}
-
-/**
- * raw_write_seqcount_barrier - do a seq write barrier
- * @s: pointer to seqcount_t
- *
- * This can be used to provide an ordering guarantee instead of the
- * usual consistency guarantee. It is one wmb cheaper, because we can
- * collapse the two back-to-back wmb()s.
- *
- * seqcount_t seq;
- * bool X = true, Y = false;
- *
- * void read(void)
- * {
- * bool x, y;
- *
- * do {
- * int s = read_seqcount_begin(&seq);
- *
- * x = X; y = Y;
- *
- * } while (read_seqcount_retry(&seq, s));
- *
- * BUG_ON(!x && !y);
- * }
- *
- * void write(void)
- * {
- * Y = true;
- *
- * raw_write_seqcount_barrier(seq);
- *
- * X = false;
- * }
- */
-static inline void raw_write_seqcount_barrier(seqcount_t *s)
-{
- s->sequence++;
+ atomic_add_int(&s->sequence, 1);
+}
+
+static inline int
+raw_read_seqcount_latch(seqcount_t *s)
+{
+ return (s->sequence);
+}
+
+static inline void
+raw_write_seqcount_latch(seqcount_t *s)
+{
smp_wmb();
- s->sequence++;
-}
-
-static inline int raw_read_seqcount_latch(seqcount_t *s)
-{
- return lockless_dereference(s->sequence);
-}
-
-/**
- * raw_write_seqcount_latch - redirect readers to even/odd copy
- * @s: pointer to seqcount_t
- *
- * The latch technique is a multiversion concurrency control method that allows
- * queries during non-atomic modifications. If you can guarantee queries never
- * interrupt the modification -- e.g. the concurrency is strictly between CPUs
- * -- you most likely do not need this.
- *
- * Where the traditional RCU/lockless data structures rely on atomic
- * modifications to ensure queries observe either the old or the new state the
- * latch allows the same for non-atomic updates. The trade-off is doubling the
- * cost of storage; we have to maintain two copies of the entire data
- * structure.
- *
- * Very simply put: we first modify one copy and then the other. This ensures
- * there is always one copy in a stable state, ready to give us an answer.
- *
- * The basic form is a data structure like:
- *
- * struct latch_struct {
- * seqcount_t seq;
- * struct data_struct data[2];
- * };
- *
- * Where a modification, which is assumed to be externally serialized, does the
- * following:
- *
- * void latch_modify(struct latch_struct *latch, ...)
- * {
- * smp_wmb(); <- Ensure that the last data[1] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
- *
- * modify(latch->data[0], ...);
- *
- * smp_wmb(); <- Ensure that the data[0] update is visible
- * latch->seq++;
- * smp_wmb(); <- Ensure that the seqcount update is visible
- *
- * modify(latch->data[1], ...);
- * }
- *
- * The query will have a form like:
- *
- * struct entry *latch_query(struct latch_struct *latch, ...)
- * {
- * struct entry *entry;
- * unsigned seq, idx;
- *
- * do {
- * seq = lockless_dereference(latch->seq);
- *
- * idx = seq & 0x01;
- * entry = data_query(latch->data[idx], ...);
- *
- * smp_rmb();
- * } while (seq != latch->seq);
- *
- * return entry;
- * }
- *
- * So during the modification, queries are first redirected to data[1]. Then we
- * modify data[0]. When that is complete, we redirect queries back to data[0]
- * and we can modify data[1].
- *
- * NOTE: The non-requirement for atomic modifications does _NOT_ include
- * the publishing of new entries in the case where data is a dynamic
- * data structure.
- *
- * An iteration might start in data[0] and get suspended long enough
- * to miss an entire modification sequence, once it resumes it might
- * observe the new entry.
- *
- * NOTE: When data is a dynamic data structure; one should use regular RCU
- * patterns to manage the lifetimes of the objects within.
- */
-static inline void raw_write_seqcount_latch(seqcount_t *s)
-{
- smp_wmb(); /* prior stores before incrementing "sequence" */
- s->sequence++;
- smp_wmb(); /* increment "sequence" before following stores */
-}
-
-/*
- * Sequence counter only version assumes that callers are using their
- * own mutexing.
- */
-static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+ atomic_add_int(&s->sequence, 1);
+}
+
+static inline void
+write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
-#endif
}
-static inline void write_seqcount_begin(seqcount_t *s)
+static inline void
+write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
-static inline void write_seqcount_end(seqcount_t *s)
+static inline void
+write_seqcount_end(seqcount_t *s)
{
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- seqcount_release(&s->dep_map, 1, _RET_IP_);
-#endif
raw_write_seqcount_end(s);
}
-/**
- * write_seqcount_invalidate - invalidate in-progress read-side seq operations
- * @s: pointer to seqcount_t
- *
- * After write_seqcount_invalidate, no read-side seq operations will complete
- * successfully and see data older than this.
- */
-static inline void write_seqcount_invalidate(seqcount_t *s)
+static inline void
+write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
- s->sequence+=2;
+ atomic_add_int(&s->sequence, 2);
}
typedef struct {
@@ -412,89 +150,87 @@ typedef struct {
spinlock_t lock;
} seqlock_t;
-/*
- * These macros triggered gcc-3.x compile-time problems. We think these are
- * OK now. Be cautious.
- */
-#define __SEQLOCK_UNLOCKED(lockname) \
+#define __SEQLOCK_UNLOCKED(lockname) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
-#define seqlock_init(x) \
+#define seqlock_init(x) \
do { \
seqcount_init(&(x)->seqcount); \
spin_lock_init(&(x)->lock); \
} while (0)
-#define DEFINE_SEQLOCK(x) \
- seqlock_t x = __SEQLOCK_UNLOCKED(x)
+#define DEFINE_SEQLOCK(x) \
+ seqlock_t x = __SEQLOCK_UNLOCKED(x)
+
-/*
- * Read side functions for starting and finalizing a read side section.
- */
-static inline unsigned read_seqbegin(seqlock_t *sl)
+static inline unsigned
+read_seqbegin(seqlock_t *sl)
{
- return read_seqcount_begin(&sl->seqcount);
+ return (read_seqcount_begin(&sl->seqcount));
}
-static inline unsigned read_seqretry(seqlock_t *sl, unsigned start)
+static inline unsigned
+read_seqretry(seqlock_t *sl, unsigned start)
{
- return read_seqcount_retry(&sl->seqcount, start);
+ return (read_seqcount_retry(&sl->seqcount, start));
}
-/*
- * Lock out other writers and update the count.
- * Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
- */
-static inline void write_seqlock(seqlock_t *sl)
+static inline void
+write_seqlock(seqlock_t *sl)
{
spin_lock(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
-static inline void write_sequnlock(seqlock_t *sl)
+static inline void
+write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
-static inline void write_seqlock_bh(seqlock_t *sl)
+static inline void
+write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
-static inline void write_sequnlock_bh(seqlock_t *sl)
+static inline void
+write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
-static inline void write_seqlock_irq(seqlock_t *sl)
+static inline void
+write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
-static inline void write_sequnlock_irq(seqlock_t *sl)
+static inline void
+write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
-static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
+static inline unsigned long
+__write_seqlock_irqsave(seqlock_t *sl)
{
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
write_seqcount_begin(&sl->seqcount);
- return flags;
+ return (flags);
}
-#define write_seqlock_irqsave(lock, flags) \
+#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
static inline void
@@ -504,79 +240,74 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
spin_unlock_irqrestore(&sl->lock, flags);
}
-/*
- * A locking reader exclusively locks out other writers and locking readers,
- * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
- * Don't need preempt_disable() because that is in the spin_lock already.
- */
-static inline void read_seqlock_excl(seqlock_t *sl)
+static inline void
+read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
-static inline void read_sequnlock_excl(seqlock_t *sl)
+static inline void
+read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
-/**
- * read_seqbegin_or_lock - begin a sequence number check or locking block
- * @lock: sequence lock
- * @seq : sequence number to be checked
- *
- * First try it once optimistically without taking the lock. If that fails,
- * take the lock. The sequence number is also used as a marker for deciding
- * whether to be a reader (even) or writer (odd).
- * N.B. seq must be initialized to an even number to begin with.
- */
-static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
+static inline void
+read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
- if (!(*seq & 1)) /* Even */
+ if (!(*seq & 1)) /* Even */
*seq = read_seqbegin(lock);
- else /* Odd */
+ else /* Odd */
read_seqlock_excl(lock);
}
-static inline int need_seqretry(seqlock_t *lock, int seq)
+static inline int
+need_seqretry(seqlock_t *lock, int seq)
{
- return !(seq & 1) && read_seqretry(lock, seq);
+ return (!(seq & 1) && read_seqretry(lock, seq));
}
-static inline void done_seqretry(seqlock_t *lock, int seq)
+static inline void
+done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
-static inline void read_seqlock_excl_bh(seqlock_t *sl)
+static inline void
+read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
-static inline void read_sequnlock_excl_bh(seqlock_t *sl)
+static inline void
+read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
-static inline void read_seqlock_excl_irq(seqlock_t *sl)
+static inline void
+read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
-static inline void read_sequnlock_excl_irq(seqlock_t *sl)
+static inline void
+read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
}
-static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
+static inline unsigned long
+__read_seqlock_excl_irqsave(seqlock_t *sl)
{
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
- return flags;
+ return (flags);
}
-#define read_seqlock_excl_irqsave(lock, flags) \
+#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
static inline void
@@ -590,12 +321,11 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
unsigned long flags = 0;
- if (!(*seq & 1)) /* Even */
+ if (!(*seq & 1)) /* Even */
*seq = read_seqbegin(lock);
- else /* Odd */
+ else /* Odd */
read_seqlock_excl_irqsave(lock, flags);
-
- return flags;
+ return (flags);
}
static inline void
@@ -604,4 +334,5 @@ done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
if (seq & 1)
read_sequnlock_excl_irqrestore(lock, flags);
}
-#endif /* __LINUX_SEQLOCK_H */
+
+#endif /* __LINUX_SEQLOCK_H */
--------------548D5A39F27368510CB30DAE--
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