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SubjectLockdep complaint in jffs2
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Hi David !

I noticed this on my BMC when building OpenBMC with Lockdep...
something worth investigating/digging ?

Cheers,
Ben.

24.068677] ======================================================
[ 24.074871] WARNING: possible circular locking dependency detected
[ 24.081065] 4.13.16-01426-gaa61047 #61 Not tainted
[ 24.085858] ------------------------------------------------------
[ 24.092039] systemd-tmpfile/616 is trying to acquire lock:
[ 24.097524] (&f->sem){+.+.+.}, at: [<802cc018>] jffs2_readpage+0x30/0x5c
[ 24.104389]
[ 24.104389] but task is already holding lock:
[ 24.110228] (&mm->mmap_sem){++++++}, at: [<8010d274>] do_page_fault+0xa0/0x3c0
[ 24.117588]
[ 24.117588] which lock already depends on the new lock.
[ 24.117588]
[ 24.125769]
[ 24.125769] the existing dependency chain (in reverse order) is:
[ 24.133246]
[ 24.133246] -> #1 (&mm->mmap_sem){++++++}:
[ 24.138878] __might_fault+0x8c/0xc0
[ 24.143007] filldir64+0x80/0x33c
[ 24.146878] jffs2_readdir+0xd8/0x1d4
[ 24.151082] iterate_dir+0x84/0x180
[ 24.155108] SyS_getdents64+0x94/0x1d8
[ 24.159400] ret_fast_syscall+0x0/0x1c
[ 24.163677]
[ 24.163677] -> #0 (&f->sem){+.+.+.}:
[ 24.168779] lock_acquire+0xe8/0x208
[ 24.172898] __mutex_lock+0x90/0x660
[ 24.177007] mutex_lock_nested+0x2c/0x34
[ 24.181472] jffs2_readpage+0x30/0x5c
[ 24.185678] filemap_fault+0x4e8/0x614
[ 24.189971] __do_fault+0x28/0xd0
[ 24.193823] handle_mm_fault+0x948/0xfb0
[ 24.198286] do_page_fault+0x2d8/0x3c0
[ 24.202573] do_DataAbort+0x50/0xe8
[ 24.206605] __dabt_usr+0x44/0x60
[ 24.210456] 0x76cf9b04
[ 24.213432]
[ 24.213432] other info that might help us debug this:
[ 24.213432]
[ 24.221439] Possible unsafe locking scenario:
[ 24.221439]
[ 24.227357] CPU0 CPU1
[ 24.231891] ---- ----
[ 24.236421] lock(&mm->mmap_sem);
[ 24.239837] lock(&f->sem);
[ 24.245244] lock(&mm->mmap_sem);
[ 24.251172] lock(&f->sem);
[ 24.254066]
[ 24.254066] *** DEADLOCK ***
[ 24.254066]
[ 24.259992] 1 lock held by systemd-tmpfile/616:
[ 24.264518] #0: (&mm->mmap_sem){++++++}, at: [<8010d274>] do_page_fault+0xa0/0x3c0
[ 24.272315]
[ 24.272315] stack backtrace:
[ 24.276692] CPU: 0 PID: 616 Comm: systemd-tmpfile Not tainted 4.13.16-01426-gaa61047 #61
[ 24.284776] Hardware name: Generic DT based system
[ 24.289610] [<8010a494>] (unwind_backtrace) from [<80107510>] (show_stack+0x20/0x24)
[ 24.297402] [<80107510>] (show_stack) from [<80612710>] (dump_stack+0x20/0x28)
[ 24.304664] [<80612710>] (dump_stack) from [<80152be8>] (print_circular_bug+0x1bc/0x2fc)
[ 24.312779] [<80152be8>] (print_circular_bug) from [<80156fcc>] (__lock_acquire+0x1674/0x18b8)
[ 24.321415] [<80156fcc>] (__lock_acquire) from [<80157d5c>] (lock_acquire+0xe8/0x208)
[ 24.329273] [<80157d5c>] (lock_acquire) from [<8062ba68>] (__mutex_lock+0x90/0x660)
[ 24.336956] [<8062ba68>] (__mutex_lock) from [<8062cde4>] (mutex_lock_nested+0x2c/0x34)
[ 24.345000] [<8062cde4>] (mutex_lock_nested) from [<802cc018>] (jffs2_readpage+0x30/0x5c)
[ 24.353217] [<802cc018>] (jffs2_readpage) from [<801f81a4>] (filemap_fault+0x4e8/0x614)
[ 24.361265] [<801f81a4>] (filemap_fault) from [<802279c4>] (__do_fault+0x28/0xd0)
[ 24.368788] [<802279c4>] (__do_fault) from [<8022b198>] (handle_mm_fault+0x948/0xfb0)
[ 24.376649] [<8022b198>] (handle_mm_fault) from [<8010d4ac>] (do_page_fault+0x2d8/0x3c0)
[ 24.384764] [<8010d4ac>] (do_page_fault) from [<80101314>] (do_DataAbort+0x50/0xe8)
[ 24.392461] [<80101314>] (do_DataAbort) from [<80108724>] (__dabt_usr+0x44/0x60)
[ 24.399867] Exception stack(0x958e5fb0 to 0x958e5ff8)
[ 24.404932] 5fa0: 02428220 76ff62d4 76ff6000 76cf9af4
[ 24.413117] 5fc0: 02428220 76dccc8c 02428220 02427e18 0000000a 02427e18 00000000 00000000
[ 24.421305] 5fe0: fbada489 7e89b1a8 76cfba2c 76cf9b04 80000010 ffffffff

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