6.1-stable review patch. If anyone has any objections, please let me know.
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From: Jens Axboe axboe@kernel.dk
[ Upstream commit 1d60d74e852647255bd8e76f5a22dc42531e4389 ]
When io_uring starts a write, it'll call kiocb_start_write() to bump the super block rwsem, preventing any freezes from happening while that write is in-flight. The freeze side will grab that rwsem for writing, excluding any new writers from happening and waiting for existing writes to finish. But io_uring unconditionally uses kiocb_start_write(), which will block if someone is currently attempting to freeze the mount point. This causes a deadlock where freeze is waiting for previous writes to complete, but the previous writes cannot complete, as the task that is supposed to complete them is blocked waiting on starting a new write. This results in the following stuck trace showing that dependency with the write blocked starting a new write:
task:fio state:D stack:0 pid:886 tgid:886 ppid:876 Call trace: __switch_to+0x1d8/0x348 __schedule+0x8e8/0x2248 schedule+0x110/0x3f0 percpu_rwsem_wait+0x1e8/0x3f8 __percpu_down_read+0xe8/0x500 io_write+0xbb8/0xff8 io_issue_sqe+0x10c/0x1020 io_submit_sqes+0x614/0x2110 __arm64_sys_io_uring_enter+0x524/0x1038 invoke_syscall+0x74/0x268 el0_svc_common.constprop.0+0x160/0x238 do_el0_svc+0x44/0x60 el0_svc+0x44/0xb0 el0t_64_sync_handler+0x118/0x128 el0t_64_sync+0x168/0x170 INFO: task fsfreeze:7364 blocked for more than 15 seconds. Not tainted 6.12.0-rc5-00063-g76aaf945701c #7963
with the attempting freezer stuck trying to grab the rwsem:
task:fsfreeze state:D stack:0 pid:7364 tgid:7364 ppid:995 Call trace: __switch_to+0x1d8/0x348 __schedule+0x8e8/0x2248 schedule+0x110/0x3f0 percpu_down_write+0x2b0/0x680 freeze_super+0x248/0x8a8 do_vfs_ioctl+0x149c/0x1b18 __arm64_sys_ioctl+0xd0/0x1a0 invoke_syscall+0x74/0x268 el0_svc_common.constprop.0+0x160/0x238 do_el0_svc+0x44/0x60 el0_svc+0x44/0xb0 el0t_64_sync_handler+0x118/0x128 el0t_64_sync+0x168/0x170
Fix this by having the io_uring side honor IOCB_NOWAIT, and only attempt a blocking grab of the super block rwsem if it isn't set. For normal issue where IOCB_NOWAIT would always be set, this returns -EAGAIN which will have io_uring core issue a blocking attempt of the write. That will in turn also get completions run, ensuring forward progress.
Since freezing requires CAP_SYS_ADMIN in the first place, this isn't something that can be triggered by a regular user.
Cc: stable@vger.kernel.org # 5.10+ Reported-by: Peter Mann peter.mann@sh.cz Link: https://lore.kernel.org/io-uring/38c94aec-81c9-4f62-b44e-1d87f5597644@sh.cz Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/rw.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)
diff --git a/io_uring/rw.c b/io_uring/rw.c index c15c7873813b3..9d6e17a244ae7 100644 --- a/io_uring/rw.c +++ b/io_uring/rw.c @@ -839,6 +839,25 @@ int io_read(struct io_kiocb *req, unsigned int issue_flags) return kiocb_done(req, ret, issue_flags); }
+static bool io_kiocb_start_write(struct io_kiocb *req, struct kiocb *kiocb) +{ + struct inode *inode; + bool ret; + + if (!(req->flags & REQ_F_ISREG)) + return true; + if (!(kiocb->ki_flags & IOCB_NOWAIT)) { + kiocb_start_write(kiocb); + return true; + } + + inode = file_inode(kiocb->ki_filp); + ret = sb_start_write_trylock(inode->i_sb); + if (ret) + __sb_writers_release(inode->i_sb, SB_FREEZE_WRITE); + return ret; +} + int io_write(struct io_kiocb *req, unsigned int issue_flags) { struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw); @@ -892,8 +911,8 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags) return ret; }
- if (req->flags & REQ_F_ISREG) - kiocb_start_write(kiocb); + if (unlikely(!io_kiocb_start_write(req, kiocb))) + return -EAGAIN; kiocb->ki_flags |= IOCB_WRITE;
if (likely(req->file->f_op->write_iter))