+ David (maintainer), linux-fsdevel, and others On Wed, Jan 27, 2016 at 04:05:35PM +0000, Joakim Tjernlund wrote: > On Wed, 2016-01-27 at 16:36 +0100, Szabó Tamás wrote: > > Hello all, > > > > I work on an embedded system running Linux 3.10 and found a deadlock > > situation between jffs2_readpage and jffs2_write. > > The problem is present on the latest 4.4 kernel too and occurs when > > two tasks want to access the same file, one reads and the other writes it. > > > > The kernel stack traces for writer and reader in deadlock: > > > > __switch_to+0x4c/0x98 > > sleep_on_page+0x10/0x24 > > __lock_page+0x8c/0x9c > > find_lock_page+0x7c/0x94 > > grab_cache_page_write_begin+0x64/0xd8 > > jffs2_write_begin+0x6c/0x2ec > > generic_file_buffered_write+0x188/0x258 > > __generic_file_aio_write+0x1e0/0x484 > > generic_file_aio_write+0x70/0xfc > > do_sync_write+0x7c/0xd4 > > vfs_write+0xc8/0x1b0 > > SyS_write+0x4c/0xa8 > > ret_from_syscall+0x0/0x38 > > > > __switch_to+0x4c/0x98 > > jffs2_readpage+0x28/0x5c > > generic_file_aio_read+0x22c/0x7a0 > > do_sync_read+0x7c/0xd4 > > vfs_read+0xb0/0x170 > > SyS_read+0x4c/0xa8 > > ret_from_syscall+0x0/0x38 > > > > The root cause here is the locking order of f->sem mutex and pagelock. > > jffs2_readpage function gets the page in locked state and then locks > > the f->sem mutex, while jffs2_write_begin does it in reverse order. > > > > I found a commit that brought in this bug. > > That was a fix for another deadlock issue: > > https://github.com/torvalds/linux/commit/5ffd3412ae5536a4c57469cb8ea31887121dcb2e > > > > According to this commit and my code inspections the lock orders may be > > the following: > > readpage: page lock, f->sem > > writepage_begin: f->sem, page lock > > writepage_end: page lock, f->sem > > GC: f->sem, page lock > > I am not sure if this is the first time I hear this or if someone else has reported > a similar issue. No, I'm pretty sure this is not the first report. I think there have even been patches. The problem is that JFFS2 is effectively unmaintained, despite what MAINTAINERS has to say about it. Previous reports: Subject: Another JFFS2 deadlock, kernel 3.4.11 http://thread.gmane.org/gmane.linux.drivers.mtd/62523 Subject: [JFFS2] Revision "jffs2: Fix lock acquisition order bug in jffs2_write_begin" introduces another dead lock. http://thread.gmane.org/gmane.linux.drivers.mtd/47986 There are other reports of deadlocks in jffs2_readpage, but in my limited scanning, they look slightly different, so I won't include them in this list. For reference: outstanding patches, waiting for a maintainer (I've been keeping patchwork up-to-date, mostly, but I'm not touching JFFS2 myself, for the most part): http://patchwork.ozlabs.org/project/linux-mtd/list/?q=jffs2 I'm tempted to resurrect this patch, to mark JFFS2 as Orphaned / Obsolete: http://patchwork.ozlabs.org/patch/422160/ David, can you please clarify your role here? Are you maintaining JFFS2 or not? Or perhaps someone else should be added? I don't really know any interested parties. Maybe the MAINTAINERS entry should be directed to linux-fsdevel too? Brian -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
