Date: Wed, 03 Feb 2010 12:05:02 +0800 From: David Xu <davidxu@freebsd.org> To: Justin Teller <justin.teller@gmail.com> Cc: freebsd-current@freebsd.org Subject: Re: Bug in kern_umtx.c -- read-write locks Message-ID: <4B68F5EE.9060606@freebsd.org> In-Reply-To: <c47019cc1002021207rc9ca100ga0dd865febdb0014@mail.gmail.com> References: <c47019cc1002021207rc9ca100ga0dd865febdb0014@mail.gmail.com>
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Justin Teller wrote: > I was working on a highly threaded app (125+ threads) that was using > the pthread rw locks, and we were stalling at strange times. After a > lot of debugging in our app, we found that a call to > pthread_rwlock_wrlock() would sometimes never return -- it seemed like > a wakeup was lost. After we convinced ourselves the bug wasn't in the > app's locking code, I started digging into the kernel. I found that > there is an issue where a wakeup can be "lost" when a thread goes to > sleep calling pthread_rwlock_wrlock. The issue is in the file > kern_umtx.c in the function do_rw_wrlock(): the code busies the lock > before sleeping, but when it tries to set the waiters bit, it's > looking at at old value (from the "try-lock" just before the busy). > This allows a race where a thread can go to sleep w/o setting the > waiters bit. Then the last thread to unlock won't wakeup the sleeping > thread. The patch below (based off of 8.0 release) fixes my problem > for the write lock and should fix the complimentary issue in > do_rw_rdlock. > > <snip> Committed, thanks!
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