Date: Wed, 19 Jan 2022 00:08:25 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Vladimir Kondratyev <vladimir@kondratyev.su> Cc: Vladimir Kondratyev <wulf@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 02ea6033020e - main - LinuxKPI: Allow spin_lock_irqsave to be called within a critical section Message-ID: <Yec6WdXUQcbZZeQF@kib.kiev.ua> In-Reply-To: <a04adcb8-3b97-5e09-d574-189b78e41bef@kondratyev.su> References: <202201182015.20IKFaWL053942@gitrepo.freebsd.org> <YechbCTSWuUs%2BNr5@kib.kiev.ua> <540a6a93-3101-02e8-b86a-50caa19f9653@kondratyev.su> <Yec1wpjqio1OwaEm@kib.kiev.ua> <a04adcb8-3b97-5e09-d574-189b78e41bef@kondratyev.su>
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On Wed, Jan 19, 2022 at 01:01:45AM +0300, Vladimir Kondratyev wrote: > On 19.01.2022 00:48, Konstantin Belousov wrote: > > On Wed, Jan 19, 2022 at 12:35:41AM +0300, Vladimir Kondratyev wrote: > > > On 18.01.2022 23:22, Konstantin Belousov wrote: > > > > On Tue, Jan 18, 2022 at 08:15:36PM +0000, Vladimir Kondratyev wrote: > > > > > The branch main has been updated by wulf: > > > > > > > > > > URL: https://cgit.FreeBSD.org/src/commit/?id=02ea6033020e11afec6472bf560b0ddebd0fa97a > > > > > > > > > > commit 02ea6033020e11afec6472bf560b0ddebd0fa97a > > > > > Author: Vladimir Kondratyev <wulf@FreeBSD.org> > > > > > AuthorDate: 2022-01-18 20:14:12 +0000 > > > > > Commit: Vladimir Kondratyev <wulf@FreeBSD.org> > > > > > CommitDate: 2022-01-18 20:14:12 +0000 > > > > > > > > > > LinuxKPI: Allow spin_lock_irqsave to be called within a critical section > > > > > with spinning on spin_trylock. dma-buf part of drm-kmod depends on this > > > > > property and absence of it support results in "mi_switch: switch in a > > > > > critical section" assertions [1][2]. > > > > > [1] https://github.com/freebsd/drm-kmod/issues/116 > > > > > [2] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261166 > > > > > MFC after: 1 week > > > > > Reviewed by: manu > > > > > Differential Revision: https://reviews.freebsd.org/D33887 > > > > > --- > > > > > .../linuxkpi/common/include/linux/spinlock.h | 27 ++++++++++++++++++---- > > > > > 1 file changed, 23 insertions(+), 4 deletions(-) > > > > > > > > > > diff --git a/sys/compat/linuxkpi/common/include/linux/spinlock.h b/sys/compat/linuxkpi/common/include/linux/spinlock.h > > > > > index a87cb7180b28..31d47fa73986 100644 > > > > > --- a/sys/compat/linuxkpi/common/include/linux/spinlock.h > > > > > +++ b/sys/compat/linuxkpi/common/include/linux/spinlock.h > > > > > @@ -37,6 +37,7 @@ > > > > > #include <sys/lock.h> > > > > > #include <sys/mutex.h> > > > > > #include <sys/kdb.h> > > > > > +#include <sys/proc.h> > > > > > #include <linux/compiler.h> > > > > > #include <linux/rwlock.h> > > > > > @@ -117,14 +118,32 @@ typedef struct { > > > > > local_bh_disable(); \ > > > > > } while (0) > > > > > -#define spin_lock_irqsave(_l, flags) do { \ > > > > > - (flags) = 0; \ > > > > > - spin_lock(_l); \ > > > > > +#define __spin_trylock_nested(_l, _n) ({ \ > > > > > + int __ret; \ > > > > > + if (SPIN_SKIP()) { \ > > > > > + __ret = 1; \ > > > > > + } else { \ > > > > > + __ret = mtx_trylock_flags(&(_l)->m, MTX_DUPOK); \ > > > > > + if (likely(__ret != 0)) \ > > > > > + local_bh_disable(); \ > > > > > + } \ > > > > > + __ret; \ > > > > > +}) > > > > > + > > > > > +#define spin_lock_irqsave(_l, flags) do { \ > > > > > + (flags) = 0; \ > > > > > + if (unlikely(curthread->td_critnest != 0)) \ > > > > > + while (!spin_trylock(_l)) {} \ > > > > > + else \ > > > > > + spin_lock(_l); \ > > > > > } while (0) > > > > > #define spin_lock_irqsave_nested(_l, flags, _n) do { \ > > > > > (flags) = 0; \ > > > > > - spin_lock_nested(_l, _n); \ > > > > > + if (unlikely(curthread->td_critnest != 0)) \ > > > > > + while (!__spin_trylock_nested(_l, _n)) {} \ > > > > > + else \ > > > > > + spin_lock_nested(_l, _n); \ > > > > > } while (0) > > > > > #define spin_unlock_irqrestore(_l, flags) do { \ > > > > You are spin-waiting for blockable mutex, am I right? > > > > > > Both, yes and no. On Linux spin_lock_irqsave is generally unblockable as it > > > disables preemption and interrupts while our version does not do this as > > > LinuxKPI is not ready for such a tricks. > > > It seems that we should explicitly add critical_enter()/critical_exit calls > > > to related dma-buf parts to make it unblockable too. > > LinuxKPI does +1 to the level of locks comparing with Linux, so their spinlocks > > become our blockable mutexes. > > > > Can you please explain why dmabufs need critical section? What is > > achieved there by disabled preemption? > > > > dma-buf uses sequence locks for synchronization. If seqlock is taken for > write, than thread it holding enters in to critical section to not force > readers to spin if writer is preempted. Unfortunately, dma-buf writers > execute callbacks which requires locks and spin_lock_irqsave is used for > synchronize these callbacks. Then, it seems that locking should be changed either to rwlocks or rmlocks, not sure which. Do you mean our seqlocks as presented in sys/seqc.h, or something Linuxish?
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