From: Eric Sandeen Subject: Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal Date: Tue, 10 Jan 2012 14:20:23 -0600 Message-ID: <4F0C9D87.8010006@sandeen.net> References: <1323367477-21685-1-git-send-email-kamal@canonical.com> <1323367477-21685-2-git-send-email-kamal@canonical.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Jan Kara , Alexander Viro , Andreas Dilger , Matthew Wilcox , Randy Dunlap , Theodore Tso , linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Surbhi Palande , Valerie Aurora , Christopher Chaltain , "Peter M. Petrakis" , Mikulas Patocka , Surbhi Palande To: Kamal Mostafa Return-path: In-Reply-To: <1323367477-21685-2-git-send-email-kamal@canonical.com> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 12/8/11 12:04 PM, Kamal Mostafa wrote: > From: Surbhi Palande > > The journal should be frozen when a filesystem freezes. What this means is > that until the filesystem is thawed again, no new transactions should be > accepted by the journal. When the filesystem thaws, inturn it should thaw the > journal and this should allow the journal to resume accepting new > transactions. While the the filesystem has frozen the journal, the clients of > the journal on calling jbd2_journal_start() will sleep on a wait queue. > Thawing the journal will wake up the sleeping clients and journalling can > progress normally. > > An example of the race condition that can happen without this patch is as > follows: > > Say the filesystem is thawed when we begin. Let tx be the time at unit x > > P1: Process doing an aio write > t1) ext4_file_write() > t2) generic_file_aio_write() > t3) __generic_file_aio_write() > // filesystem is not frozen, so we do not block in the next check. > t4) vfs_check_frozen() > t5) generic_write_checks() > ----------------- Prempted------------------ > > P2: Process that does filesystem freeze > > t6) freeze_super() > t7) sync_filesystem() > t8) sync_blockdev() > t9) sb->s_op->freeze_fs() (= ext4_freeze) > t10) jbd2_journal_lock_updates() > t11) jbd2_journal_flush() > // Need to unlock the journal before returning to user space. > t12) jbd2_journal_unlock_updates() > // Journal is unlocked and so we can start accepting new transactions now. > > // freezing process completes execution. Page cache is now clean and should > // remain clean till the filesystem is frozen. > -------------------------------------------- > > P1: writing process gets the control back > t13) generic_file_buffered_write() > t14) generic_perform_write() > t15) a_ops->write_begin() (= ext4_write_begin) > t16) ext4_journal_start() > // New handle is started. We do not block here! Write continues > // dirtying the page cache while the filesystem is frozen! Hrm let me think through this a little more; we actually do: t16) ext4_journal_start() t17) ext4_journal_start_sb() t18) handle = ext4_journal_current_handle(); t19) if (!handle) vfs_check_frozen() t20) ... jbd2_journal_start() So actually we *do* block new handles, but let *existing* ones continue (see commits 6b0310fbf087ad6e9e3b8392adca97cd77184084 and be4f27d324e8ddd57cc0d4d604fe85ee0425cba9) So your assertion that a new handle is started is incorrect in general, isn't it? So then does the fix seem necessary? Or, at least, in the fashion below - maybe we need to just make sure all started handles complete before the unlock_updates? Or am I missing something...? -Eric > > BugLink: https://bugs.launchpad.net/bugs/897421 > Signed-off-by: Surbhi Palande > Acked-by: Jan Kara > Reviewed-by: Andreas Dilger > Cc: Kamal Mostafa > Tested-by: Peter M. Petrakis > Signed-off-by: Kamal Mostafa > --- > fs/jbd2/journal.c | 1 + > fs/jbd2/transaction.c | 42 ++++++++++++++++++++++++++++++++++++++++++ > include/linux/jbd2.h | 7 +++++++ > 3 files changed, 50 insertions(+), 0 deletions(-) > > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c > index 0fa0123..f0170cc 100644 > --- a/fs/jbd2/journal.c > +++ b/fs/jbd2/journal.c > @@ -894,6 +894,7 @@ static journal_t * journal_init_common (void) > init_waitqueue_head(&journal->j_wait_checkpoint); > init_waitqueue_head(&journal->j_wait_commit); > init_waitqueue_head(&journal->j_wait_updates); > + init_waitqueue_head(&journal->j_wait_frozen); > mutex_init(&journal->j_barrier); > mutex_init(&journal->j_checkpoint_mutex); > spin_lock_init(&journal->j_revoke_lock); > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c > index a0e41a4..340ee35 100644 > --- a/fs/jbd2/transaction.c > +++ b/fs/jbd2/transaction.c > @@ -173,6 +173,17 @@ repeat: > journal->j_barrier_count == 0); > goto repeat; > } > + /* Don't let a new handle start when a journal is frozen. > + * jbd2_journal_freeze calls jbd2_journal_unlock_updates() only after > + * the j_flags indicate that the journal is frozen. So if the > + * j_barrier_count is 0, then check if this was made 0 by the freezing > + * process > + */ > + if (journal->j_flags & JBD2_FROZEN) { > + read_unlock(&journal->j_state_lock); > + wait_event(journal->j_wait_frozen, (journal->j_flags & JBD2_FROZEN)); > + goto repeat; > + } > > if (!journal->j_running_transaction) { > read_unlock(&journal->j_state_lock); > @@ -492,6 +503,37 @@ int jbd2_journal_restart(handle_t *handle, int nblocks) > } > EXPORT_SYMBOL(jbd2_journal_restart); > > +int jbd2_journal_freeze(journal_t *journal) > +{ > + int error = 0; > + /* Now we set up the journal barrier. */ > + jbd2_journal_lock_updates(journal); > + > + /* > + * Don't clear the needs_recovery flag if we failed to flush > + * the journal. > + */ > + error = jbd2_journal_flush(journal); > + if (error >= 0) { > + write_lock(&journal->j_state_lock); > + journal->j_flags |= JBD2_FROZEN; > + write_unlock(&journal->j_state_lock); > + } > + jbd2_journal_unlock_updates(journal); > + return error; > +} > +EXPORT_SYMBOL(jbd2_journal_freeze); > + > +void jbd2_journal_thaw(journal_t * journal) > +{ > + write_lock(&journal->j_state_lock); > + journal->j_flags &= ~JBD2_FROZEN; > + write_unlock(&journal->j_state_lock); > + wake_up(&journal->j_wait_frozen); > +} > +EXPORT_SYMBOL(jbd2_journal_thaw); > + > + > /** > * void jbd2_journal_lock_updates () - establish a transaction barrier. > * @journal: Journal to establish a barrier on. > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h > index 2092ea2..bfa0752 100644 > --- a/include/linux/jbd2.h > +++ b/include/linux/jbd2.h > @@ -658,6 +658,7 @@ jbd2_time_diff(unsigned long start, unsigned long end) > * @j_wait_checkpoint: Wait queue to trigger checkpointing > * @j_wait_commit: Wait queue to trigger commit > * @j_wait_updates: Wait queue to wait for updates to complete > + * @j_wait_frozen: Wait queue to wait for journal to thaw > * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints > * @j_head: Journal head - identifies the first unused block in the journal > * @j_tail: Journal tail - identifies the oldest still-used block in the > @@ -775,6 +776,9 @@ struct journal_s > /* Wait queue to wait for updates to complete */ > wait_queue_head_t j_wait_updates; > > + /* Wait queue to wait for journal to thaw*/ > + wait_queue_head_t j_wait_frozen; > + > /* Semaphore for locking against concurrent checkpoints */ > struct mutex j_checkpoint_mutex; > > @@ -953,6 +957,7 @@ struct journal_s > #define JBD2_ABORT_ON_SYNCDATA_ERR 0x040 /* Abort the journal on file > * data write error in ordered > * mode */ > +#define JBD2_FROZEN 0x080 /* Journal thread frozen along with filesystem */ > > /* > * Function declarations for the journaling transaction and buffer > @@ -1060,6 +1065,8 @@ extern void jbd2_journal_invalidatepage(journal_t *, > struct page *, unsigned long); > extern int jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t); > extern int jbd2_journal_stop(handle_t *); > +extern int jbd2_journal_freeze(journal_t *); > +extern void jbd2_journal_thaw(journal_t *); > extern int jbd2_journal_flush (journal_t *); > extern void jbd2_journal_lock_updates (journal_t *); > extern void jbd2_journal_unlock_updates (journal_t *);