[dm-devel] [PATCH v3 02/19] libmultipath: fix tur checker double locking
Benjamin Marzinski
bmarzins at redhat.com
Thu Oct 4 16:45:21 UTC 2018
On Mon, Oct 01, 2018 at 10:09:41PM +0200, Martin Wilck wrote:
> On Fri, 2018-09-21 at 18:05 -0500, Benjamin Marzinski wrote:
> > tur_devt() locks ct->lock. However, it is ocassionally called while
> > ct->lock is already locked. In reality, there is no reason why we
> > need
> > to lock all the accesses to ct->devt. The tur checker only needs to
> > write to this variable one time, when it first gets the file
> > descripter
> > that it is checking. It also never uses ct->devt directly. Instead,
> > it
> > always graps the major and minor, and turns them into a string. This
> > patch changes ct->devt into that string, and sets it in
> > libcheck_init()
> > when it is first initializing the checker context. After that, ct-
> > >devt
> > is only ever read.
>
> I like the lock removal a lot, but not so much the conversion into a
> string. Why not keep the dev_t?
Because we will simply convert it into a string every time we use it,
instead of doing the work one time. It's 24 more bytes in the
tur_checker_context, but the code is easier to read, and we're not doing
the same work again and again.
>
> Or maybe even easier, the other way around: why don't we make it a
> char* and simply set checker->dev_t = &pp->dev_t?
The whole reason we have tur_checker_context->holders is that it's
possible for a path to be removed (or orphaned) while the thread is
still running. The tur_checker_context needs to keep all its own
storage, so that it never as to worry about pointing to freed memory.
-Ben
> Regards,
> Martin
>
> >
> > Cc: Bart Van Assche <bart.vanassche at wdc.com>4
> > Signed-off-by: Benjamin Marzinski <bmarzins at redhat.com>
> > ---
> > libmultipath/checkers/tur.c | 55 +++++++++++++--------------------
> > ------------
> > 1 file changed, 16 insertions(+), 39 deletions(-)
> >
> > diff --git a/libmultipath/checkers/tur.c
> > b/libmultipath/checkers/tur.c
> > index 275541f..d173648 100644
> > --- a/libmultipath/checkers/tur.c
> > +++ b/libmultipath/checkers/tur.c
> > @@ -37,36 +37,24 @@
> > #define MSG_TUR_FAILED "tur checker failed to initialize"
> >
> > struct tur_checker_context {
> > - dev_t devt;
> > + char devt[32];
> > int state;
> > - int running;
> > + int running; /* uatomic access only */
> > int fd;
> > unsigned int timeout;
> > time_t time;
> > pthread_t thread;
> > pthread_mutex_t lock;
> > pthread_cond_t active;
> > - int holders;
> > + int holders; /* uatomic access only */
> > char message[CHECKER_MSG_LEN];
> > };
> >
> > -static const char *tur_devt(char *devt_buf, int size,
> > - struct tur_checker_context *ct)
> > -{
> > - dev_t devt;
> > -
> > - pthread_mutex_lock(&ct->lock);
> > - devt = ct->devt;
> > - pthread_mutex_unlock(&ct->lock);
> > -
> > - snprintf(devt_buf, size, "%d:%d", major(devt), minor(devt));
> > - return devt_buf;
> > -}
> > -
> > int libcheck_init (struct checker * c)
> > {
> > struct tur_checker_context *ct;
> > pthread_mutexattr_t attr;
> > + struct stat sb;
> >
> > ct = malloc(sizeof(struct tur_checker_context));
> > if (!ct)
> > @@ -81,6 +69,9 @@ int libcheck_init (struct checker * c)
> > pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
> > pthread_mutex_init(&ct->lock, &attr);
> > pthread_mutexattr_destroy(&attr);
> > + if (fstat(c->fd, &sb) == 0)
> > + snprintf(ct->devt, sizeof(ct->devt), "%d:%d",
> > major(sb.st_rdev),
> > + minor(sb.st_rdev));
> > c->context = ct;
> >
> > return 0;
> > @@ -232,14 +223,12 @@ static void *tur_thread(void *ctx)
> > {
> > struct tur_checker_context *ct = ctx;
> > int state, running;
> > - char devt[32];
> >
> > /* This thread can be canceled, so setup clean up */
> > tur_thread_cleanup_push(ct);
> > rcu_register_thread();
> >
> > - condlog(3, "%s: tur checker starting up",
> > - tur_devt(devt, sizeof(devt), ct));
> > + condlog(3, "%s: tur checker starting up", ct->devt);
> >
> > /* TUR checker start up */
> > pthread_mutex_lock(&ct->lock);
> > @@ -256,8 +245,8 @@ static void *tur_thread(void *ctx)
> > pthread_cond_signal(&ct->active);
> > pthread_mutex_unlock(&ct->lock);
> >
> > - condlog(3, "%s: tur checker finished, state %s",
> > - tur_devt(devt, sizeof(devt), ct),
> > checker_state_name(state));
> > + condlog(3, "%s: tur checker finished, state %s", ct->devt,
> > + checker_state_name(state));
> >
> > running = uatomic_xchg(&ct->running, 0);
> > if (!running)
> > @@ -305,20 +294,12 @@ int libcheck_check(struct checker * c)
> > {
> > struct tur_checker_context *ct = c->context;
> > struct timespec tsp;
> > - struct stat sb;
> > pthread_attr_t attr;
> > int tur_status, r;
> > - char devt[32];
> >
> > if (!ct)
> > return PATH_UNCHECKED;
> >
> > - if (fstat(c->fd, &sb) == 0) {
> > - pthread_mutex_lock(&ct->lock);
> > - ct->devt = sb.st_rdev;
> > - pthread_mutex_unlock(&ct->lock);
> > - }
> > -
> > if (c->sync)
> > return tur_check(c->fd, c->timeout,
> > copy_msg_to_checker, c);
> >
> > @@ -327,8 +308,7 @@ int libcheck_check(struct checker * c)
> > */
> > r = pthread_mutex_lock(&ct->lock);
> > if (r != 0) {
> > - condlog(2, "%s: tur mutex lock failed with %d",
> > - tur_devt(devt, sizeof(devt), ct), r);
> > + condlog(2, "%s: tur mutex lock failed with %d", ct-
> > >devt, r);
> > MSG(c, MSG_TUR_FAILED);
> > return PATH_WILD;
> > }
> > @@ -338,14 +318,12 @@ int libcheck_check(struct checker * c)
> > int running = uatomic_xchg(&ct->running, 0);
> > if (running)
> > pthread_cancel(ct->thread);
> > - condlog(3, "%s: tur checker timeout",
> > - tur_devt(devt, sizeof(devt), ct));
> > + condlog(3, "%s: tur checker timeout", ct-
> > >devt);
> > ct->thread = 0;
> > MSG(c, MSG_TUR_TIMEOUT);
> > tur_status = PATH_TIMEOUT;
> > } else if (uatomic_read(&ct->running) != 0) {
> > - condlog(3, "%s: tur checker not finished",
> > - tur_devt(devt, sizeof(devt),
> > ct));
> > + condlog(3, "%s: tur checker not finished", ct-
> > >devt);
> > tur_status = PATH_PENDING;
> > } else {
> > /* TUR checker done */
> > @@ -361,7 +339,7 @@ int libcheck_check(struct checker * c)
> > if (ct->state == PATH_PENDING) {
> > pthread_mutex_unlock(&ct->lock);
> > condlog(3, "%s: tur thread not
> > responding",
> > - tur_devt(devt, sizeof(devt),
> > ct));
> > + ct->devt);
> > return PATH_TIMEOUT;
> > }
> > }
> > @@ -381,7 +359,7 @@ int libcheck_check(struct checker * c)
> > ct->thread = 0;
> > pthread_mutex_unlock(&ct->lock);
> > condlog(3, "%s: failed to start tur thread,
> > using"
> > - " sync mode", tur_devt(devt,
> > sizeof(devt), ct));
> > + " sync mode", ct->devt);
> > return tur_check(c->fd, c->timeout,
> > copy_msg_to_checker, c);
> > }
> > @@ -392,8 +370,7 @@ int libcheck_check(struct checker * c)
> > pthread_mutex_unlock(&ct->lock);
> > if (uatomic_read(&ct->running) != 0 &&
> > (tur_status == PATH_PENDING || tur_status ==
> > PATH_UNCHECKED)) {
> > - condlog(3, "%s: tur checker still running",
> > - tur_devt(devt, sizeof(devt), ct));
> > + condlog(3, "%s: tur checker still running", ct-
> > >devt);
> > tur_status = PATH_PENDING;
> > } else {
> > int running = uatomic_xchg(&ct->running, 0);
>
> --
> Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
> HRB 21284 (AG Nürnberg)
>
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