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dir_lookup, locking and EINTR

From: Samuel Thibault
Subject: dir_lookup, locking and EINTR
Date: Fri, 6 Jun 2008 00:12:31 +0100
User-agent: Mutt/1.5.12-2006-07-14


We are still facing some ext2fs hangs sometimes, and I got an
interesting backtrace today:

Thread 94 (thread 348.94):
#0  0x010d753c in mach_msg_trap () from /lib/libc.so.0.3
#1  0x010d7cc9 in mach_msg () from /lib/libc.so.0.3
#2  0x010a3630 in cproc_block ()
    at /build/mbanck/hurd-20071119/build-tree/hurd/libthreads/cprocs.c:643
#3  0x010a3d1a in __mutex_lock_solid (ptr=________ CCCC 0x8123c6c __________)
    at /build/mbanck/hurd-20071119/build-tree/hurd/libthreads/cprocs.c:955
#4  0x0104a1e9 in diskfs_release_peropen (po=0x8242640)
    at /build/mbanck/hurd-20071119/build-tree/hurd/libdiskfs/peropen-rele.c:25
#5  0x0104a402 in diskfs_protid_rele (arg=0x81d0818)
    at /build/mbanck/hurd-20071119/build-tree/hurd/libdiskfs/protid-rele.c:34
#6  0x010aa912 in _ports_complete_deallocate (pi=0x81d0818)
#7  0x010a9421 in ports_port_deref (portstruct=0x81d0818)
    at /build/mbanck/hurd-20071119/build-tree/hurd/libports/port-deref.c:48
#8  0x01038825 in diskfs_S_dir_lookup (dircred=0x80e9698, 
    path=0x1aedf4c "@test", flags=262154=O_NONBLOCK|O_WRONLY|O_EXLOCK, mode=0, 
    retryname=0x1aebf4c "", returned_port=0x1aec350, 
    at /build/mbanck/hurd-20071119/build-tree/hurd/libdiskfs/dir-lookup.c:478

dir-lookup.c:478 is as follows:

469 if (! error)
470   {
471     if (flags & O_EXLOCK)
472       error = fshelp_acquire_lock (&np->userlock, &newpi->po->lock_status,
473                                    &np->lock, LOCK_EX);
474     else if (flags & O_SHLOCK)
475       error = fshelp_acquire_lock (&np->userlock, &newpi->po->lock_status,
476                                    &np->lock, LOCK_SH);
477     if (error)
478       ports_port_deref (newpi); /* Get rid of NEWPI.  */
479   }

i.e. someone tried to open @test exclusively, but it failed (EINTR),
and thus we drop newpi.  The problem is that since that's the last
reference, it cals _ports_complete_deallocate, which ends up calling
diskfs_release_peropen which tries to acquire the lock on np, which we
_already_ have, thus the deadlock, which quickly propagates to the /.

It looks like we have the same problem with diskfs_create_protid().

Since in case of an error we don't return a locked np, I guess the
correct fix is as attached to this mail?


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