Date: Mon, 25 Jan 2010 14:56:32 -0500 From: John Baldwin <jhb@freebsd.org> To: Attilio Rao <attilio@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r202889 - head/sys/kern Message-ID: <201001251456.32459.jhb@freebsd.org> In-Reply-To: <201001231554.o0NFsMbx049837@svn.freebsd.org> References: <201001231554.o0NFsMbx049837@svn.freebsd.org>
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On Saturday 23 January 2010 10:54:22 am Attilio Rao wrote: > Author: attilio > Date: Sat Jan 23 15:54:21 2010 > New Revision: 202889 > URL: http://svn.freebsd.org/changeset/base/202889 > > Log: > - Fix a race in sched_switch() of sched_4bsd. > In the case of the thread being on a sleepqueue or a turnstile, the > sched_lock was acquired (without the aid of the td_lock interface) and > the td_lock was dropped. This was going to break locking rules on other > threads willing to access to the thread (via the td_lock interface) and > modify his flags (allowed as long as the container lock was different > by the one used in sched_switch). > In order to prevent this situation, while sched_lock is acquired there > the td_lock gets blocked. [0] > - Merge the ULE's internal function thread_block_switch() into the global > thread_lock_block() and make the former semantic as the default for > thread_lock_block(). This means that thread_lock_block() will not > disable interrupts when called (and consequently thread_unlock_block() > will not re-enabled them when called). This should be done manually > when necessary. > Note, however, that ULE's thread_unblock_switch() is not reaped > because it does reflect a difference in semantic due in ULE (the > td_lock may not be necessarilly still blocked_lock when calling this). > While asymmetric, it does describe a remarkable difference in semantic > that is good to keep in mind. Does this affect the various #ifdef's for handling the third argument to cpu_switch()? E.g. does 4BSD need to spin if td_lock is &blocked_lock? Also, BLOCK_SPIN() on x86 is non-optimal. It should not do cmpxchg in a loop. Instead, it should do cmp in a loop, and if the cmp succeeds, then try cmpxchg. -- John Baldwin
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