Date: Sat, 30 Jan 2010 01:23:22 +0200 From: Alexander Motin <mav@FreeBSD.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org, FreeBSD-Current <freebsd-current@freebsd.org>, freebsd-geom@freebsd.org Subject: Re: Deadlock between GEOM and devfs device destroy and process exit. Message-ID: <4B636DEA.2060901@FreeBSD.org> In-Reply-To: <20100129231110.GS3877@deviant.kiev.zoral.com.ua> References: <4B636812.8060403@FreeBSD.org> <20100129231110.GS3877@deviant.kiev.zoral.com.ua>
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Kostik Belousov wrote: > On Sat, Jan 30, 2010 at 12:58:26AM +0200, Alexander Motin wrote: >> Hi. >> >> Experimenting with SATA hot-plug I've found quite repeatable deadlock >> case. Problem observed when several SATA devices, opened via devfs, >> disappear at exactly same time. In my case, at time of unplugging SATA >> Port Multiplier with several disks beyond it. All I have to do is to run >> several `dd if=/dev/adaX of=/dev/null bs=1m &` commands and unplug >> multiplier. That causes predictable I/O errors and devices destruction. >> But with high probability several dd processes getting stuck in kernel. >> >> I've discovered such pieces of problem: >> - CAM receives disconnect event and starts device destruction. But as >> device is still opened, it can't do it immediately. >> - dd receives I/O error and exits. >> - exit1() call closes all descriptors, including adaX device. It >> triggers final device destruction, by sending event to geom_dev. >> >> adaclose(4571fa00,4,40c16576,76,0,...) at 0x4049c521 >> g_disk_access(457e2200,ffffffff,0,0,0,...) at 0x4080b9a4 >> g_access(45643d80,ffffffff,0,0,2000,...) at 0x40810ccb >> g_dev_close(45766500,1,2000,4569fd80,4569fd80,...) at 0x4080a425 >> devfs_close(7b604aa8,80000,457f8000,80000,7b604acc,...) at 0x407f2762 >> VOP_CLOSE_APV(40d03180,7b604aa8,40c2e681,128,0,...) at 0x40b6da55 >> vn_close(457f8000,1,45624300,4569fd80,451271e0,...) at 0x40912750 >> vn_closefile(4566da48,4569fd80,4566da48,0,7b604b58,...) at 0x40912854 >> devfs_close_f(4566da48,4569fd80,3,0,4566da48,...) at 0x407f235b >> _fdrop(4566da48,4569fd80,7b604b8c,408b5cec,0,4569fe24,40eb23a8,40d10460,40c1a8bb,4560672c,721,40c1a8b2,7b604bb4,40878220,4560672c,8,40c1a8b2,721) >> at 0x40836da3 >> closef(4566da48,4569fd80,721,71e,4569fe24,...) at 0x40838ad0 >> fdfree(4569fd80,0,40c1b1a9,107,7b604c80,...) at 0x408394da >> exit1(4569fd80,100,7b604d2c,40b565c0,4569fd80,...) at 0x40844423 >> sys_exit(4569fd80,7b604cf8,40c59d34,40c26be4,4569d2a8,...) at 0x408450fd >> syscall(7b604d38) at 0x40b565c0 >> >> - GEOM event thread tries to destroy /dev/adaX device (which should be >> already free at this moment), but for some reason freezes, waiting for >> device to be freed: >> >> 0 2 0 0 -8 0 0 8 devdrn DL ?? 0:02.89 >> [g_event] >> >> - as GEOM event is still not handled, exit1() waits for it: >> >> kdb_backtrace(40c16bc4,0,40c16ab1,56,4540e640,...) at 0x408a2909 >> g_waitidle(4569fd80,0,40c1b1a9,107,7b604c80,...) at 0x4080cd1f >> exit1(4569fd80,100,7b604d2c,40b565c0,4569fd80,...) at 0x40844431 >> sys_exit(4569fd80,7b604cf8,40c59d34,40c26be4,4569d2a8,...) at 0x408450fd >> syscall(7b604d38) at 0x40b565c0 >> >> - system stationary. GEOM frozen. No way to get out of this, except >> pushing reset. >> >> 0 1065 1055 0 44 0 5344 3040 g_wait DE 0 0:00.43 dd >> if=/dev/ada1 of=/dev/null bs=1m >> 0 1066 1055 0 44 0 5344 3040 GEOM t DE 0 0:00.07 dd >> if=/dev/ada2 of=/dev/null bs=1m >> >> >> So, does anybody have good idea why destroy_dev() can't complete? > > The devdrn state means that thread performing the device destruction, > i.e. the thread called destroy_dev(), is waiting for threads to leave > the cdevsw d_* methods. The thread that notified the destruction thread > did that from d_close() method. This resulted in the deadlock. d_close() doesn't call destroy_dev() directly. It schedules different thread to do it. destroy_dev() should run after d_close() already complete. Though I haven't checked how it is locked. > I introduced destroy_dev_sched(9) KPI to handle this and similar issues. > Note that race-free use of destroy_dev_sched(9) is quite hard. I think it should work without it here. Shouldn't it? -- Alexander Motin
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