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SubjectRe: 2.0, 2.2, 2.4, 2.5: fsync buffer race
Hi!

> mutt D 00000292 0 14627 24890 (NOTLB)
> Call Trace: [__wait_on_buffer+86/144] [ext3_free_inode+391/1248] [ll_rw_block+309/464] [ext3_find_entry+828/864] [ext3_lookup+65/208]
> [lookup_hash+194/288] [sys_unlink+139/288] [system_call+51/56]
> tee D ECD560C0 0 111 11445 110 (NOTLB)
> Call Trace: [journal_dirty_metadata+420/608] [__wait_on_buffer+86/144] [__cleanup_transaction+362/368] [journal_cancel_revoke+91/448] [log_do_checkpoint+299/448]
> [n_tty_receive_buf+400/1808] [journal_dirty_metadata+420/608] [n_tty_receive_buf+400/1808] [journal_dirty_metadata+420/608] [ext3_do_update_inode+375/992] [__jbd_kmalloc+35/176]
> [log_wait_for_space+130/144] [start_this_handle+191/864] [new_handle+75/112] [journal_start+165/208] [ext3_prepare_write+567/576] [write_chan+335/512]
> [generic_file_write_nolock+1023/2032] [pipe_wait+125/160] [generic_file_write+77/112] [ext3_file_write+57/224] [sys_write+163/336] [system_call+51/56]
> python D 00000180 0 21334 112 21336 21335 (NOTLB)
> Call Trace: [start_request+409/544] [__down+105/192] [__down_failed+8/12] [.text.lock.checkpoint+15/173] [start_this_handle+191/864]
> [new_handle+75/112] [journal_start+165/208] [ext3_lookup+0/208] [ext3_create+300/320] [vfs_create+153/304] [open_namei+1301/1456]
> [filp_open+62/112] [sys_open+83/192] [system_call+51/56]
> python D E2EB302C 2388 21335 112 21337 21334 (NOTLB)
> Call Trace: [dput+33/320] [__down+105/192] [__down_failed+8/12] [.text.lock.namei+265/988] [filp_open+62/112]
> [sys_open+83/192] [system_call+51/56]
> procmail D E51AC016 0 21378 1 19102 (NOTLB)
> Call Trace: [dput+33/320] [__down+105/192] [__down_failed+8/12] [.text.lock.namei+265/988] [filp_open+62/112]
> [sys_open+83/192] [system_call+51/56]
> python D C01186FF 0 21470 107 21471 (NOTLB)
> Call Trace: [do_page_fault+463/1530] [__down+105/192] [__jbd_kmalloc+35/176] [__down_failed+8/12] [.text.lock.checkpoint+15/173]
> [start_this_handle+191/864] [new_handle+75/112] [journal_start+165/208] [do_wp_page+416/1088] [ext3_dirty_inode+336/368] [__mark_inode_dirty+195/208]
> [update_inode_times+39/64] [generic_file_write_nolock+630/2032] [generic_file_write+77/112] [ext3_file_write+57/224] [sys_write+163/336] [system_call+51/56]
> python D C01186FF 0 21471 107 21470 (NOTLB)
> Call Trace: [do_page_fault+463/1530] [__down+105/192] [__jbd_kmalloc+35/176] [__down_failed+8/12] [.text.lock.checkpoint+15/173]
> [start_this_handle+191/864] [new_handle+75/112] [journal_start+165/208] [do_wp_page+416/1088] [ext3_dirty_inode+336/368] [__mark_inode_dirty+195/208]
> [update_inode_times+39/64] [generic_file_write_nolock+630/2032] [generic_file_write+77/112] [ext3_file_write+57/224] [sys_write+163/336] [system_call+51/56]
> python D 000001E0 0 21732 21731 (NOTLB)
> Call Trace: [bread+32/160] [__down+105/192] [__jbd_kmalloc+35/176] [__down_failed+8/12] [.text.lock.checkpoint+15/173]
> [start_this_handle+191/864] [new_handle+75/112] [journal_start+165/208] [dput+33/320] [ext3_unlink+440/448] [vfs_unlink+233/480]
> [sys_unlink+183/288] [system_call+51/56]
> procmail D D0E13016 0 21914 21913 (NOTLB)
> Call Trace: [dput+33/320] [__down+105/192] [__down_failed+8/12] [.text.lock.namei+265/988] [filp_open+62/112]
> [sys_open+83/192] [system_call+51/56]
> procmail D E39F3016 0 21999 21998 (NOTLB)
> Call Trace: [dput+33/320] [__down+105/192] [__down_failed+8/12] [.text.lock.namei+265/988] [filp_open+62/112]
> [sys_open+83/192] [system_call+51/56]

never mind, I found the real bug and it's not the fsync fix or in ext3.

This other more recent cleanedup trace made the real bug more obvious
since less stuff was involved on the fs side (because it wasn't an fs
problem after all):

gzip D CEAE5740 1616 1234 1229 (NOTLB)
Call Trace: [ext3_new_block+1062/2448] [do_schedule+284/416] [__wait_on_buffer+84/144] [__cleanup_transaction+252/256] [log_do_checkpoint+286/432]
[ide_build_sglist+156/512] [__ide_dma_begin+57/80] [__ide_dma_count+22/32] [__ide_dma_write+326/352] [dma_timer_expiry+0/224] [do_rw_disk+884/1712]
[context_switch+131/320] [ide_wait_stat+159/304] [start_request+409/544] [__jbd_kmalloc+35/176] [log_wait_for_space+108/128] [start_this_handle+184/848]
[new_handle+75/112] [journal_start+165/208] [__alloc_pages+100/624] [ext3_prepare_write+336/352] [lru_cache_add+99/112] [generic_file_write_nolock+999/1856]
[generic_file_write+76/112] [ext3_file_write+57/224] [sys_write+163/304] [system_call+51/56]
shutdown D DFE7D940 0 2308 1 2309 2169 (NOTLB)
Call Trace: [do_schedule+284/416] [__wait_on_buffer+84/144] [__refile_buffer+86/112] [wait_for_buffers+172/176] [wait_for_locked_buffers+35/48]
[sync_buffers+106/112] [fsync_dev+68/80] [sys_sync+15/32] [system_call+51/56]
sshd D D0CC93DC 0 2338 601 2339 2340 1226 (NOTLB)
Call Trace: [do_schedule+284/416] [__down+90/160] [__jbd_kmalloc+35/176] [__down_failed+8/12] [.text.lock.checkpoint+15/109]
[start_this_handle+184/848] [new_handle+75/112] [journal_start+165/208] [ext3_dirty_inode+276/288] [__mark_inode_dirty+150/160] [update_atime+94/96]
[generic_file_read+174/352] [file_read_actor+0/144] [sys_read+163/304] [sys_fstat64+73/128] [system_call+51/56]
sync D DFF858C0 80 2422 2394 (NOTLB)
Call Trace: [do_schedule+284/416] [__wait_on_buffer+84/144] [wait_for_buffers+172/176] [wait_for_locked_buffers+35/48] [sync_buffers+106/112]
[fsync_dev+68/80] [sys_sync+15/32] [system_call+51/56]

sshd waits for gzip on the journal lock, gzip and shutdown waits on a
buffer that was supposed to be submitted by `sync`, but really `sync`
never submitted it, it only locked it. This bug is present only in
2.4.21pre4aa3 (all previous didn't had this problem). This was extremely
hard to trigger, so it's not surprising few people noticed it,
expecially on fast machines. The lowlatency improvements in
write_some_buffers zeroed out 'count' at the second pass, and so we lost
track of those bh if need_resched was set during the write_some_buffers
pass. this will be fixed in my next tree that I can now release
confortably back in rock solid state with also the fsync-race fix
re-added ;)

here the untested fix against 2.4.21pre4aa3 that I will include in
2.4.21pre5aa1:

--- x/fs/buffer.c.~1~ 2003-03-13 06:15:29.000000000 +0100
+++ x/fs/buffer.c 2003-03-14 07:36:40.000000000 +0100
@@ -236,10 +236,10 @@ static int write_some_buffers(kdev_t dev
unsigned int count;
int nr, num_sched = 0;

+ count = 0;
restart:
next = lru_list[BUF_DIRTY];
nr = nr_buffers_type[BUF_DIRTY];
- count = 0;
while (next && --nr >= 0) {
struct buffer_head * bh = next;
next = bh->b_next_free;
Sorry for the noise.

Andrea
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