From 84483ea42cd4f2781d6e97a83ab3ebd0ff19fb10 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 16 Jun 2010 16:48:13 -0700 Subject: rcu: add shiny new debug assists to Documentation/RCU/checklist.txt Add a section describing PROVE_RCU, DEBUG_OBJECTS_RCU_HEAD, and the __rcu sparse checking to the RCU checklist. Suggested-by: David Miller Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett --- Documentation/RCU/checklist.txt | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) (limited to 'Documentation') diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 790d1a8..c7c6788 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -365,3 +365,26 @@ over a rather long period of time, but improvements are always welcome! and the compiler to freely reorder code into and out of RCU read-side critical sections. It is the responsibility of the RCU update-side primitives to deal with this. + +17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and + the __rcu sparse checks to validate your RCU code. These + can help find problems as follows: + + CONFIG_PROVE_RCU: check that accesses to RCU-protected data + structures are carried out under the proper RCU + read-side critical section, while holding the right + combination of locks, or whatever other conditions + are appropriate. + + CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the + same object to call_rcu() (or friends) before an RCU + grace period has elapsed since the last time that you + passed that same object to call_rcu() (or friends). + + __rcu sparse checks: tag the pointer to the RCU-protected data + structure with __rcu, and sparse will warn you if you + access that pointer without the services of one of the + variants of rcu_dereference(). + + These debugging aids can help you find problems that are + otherwise extremely difficult to spot. -- cgit v1.1 From 65e423f8ee5843e1ea3f2d94adf4ba3560a17f7b Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 19 May 2010 10:42:16 -0700 Subject: Update documentation to note the passage of INIT_RCU_HEAD() Signed-off-by: Alexey Dobriyan Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett --- Documentation/DocBook/kernel-locking.tmpl | 8 -------- 1 file changed, 8 deletions(-) (limited to 'Documentation') diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl index 084f6ad..e6cc574 100644 --- a/Documentation/DocBook/kernel-locking.tmpl +++ b/Documentation/DocBook/kernel-locking.tmpl @@ -1725,14 +1725,6 @@ the amount of locking which needs to be done. if (++cache_num > MAX_CACHE_SIZE) { struct object *i, *outcast = NULL; list_for_each_entry(i, &cache, list) { -@@ -85,6 +94,7 @@ - obj->popularity = 0; - atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ - spin_lock_init(&obj->lock); -+ INIT_RCU_HEAD(&obj->rcu); - - spin_lock_irqsave(&cache_lock, flags); - __cache_add(obj); @@ -104,12 +114,11 @@ struct object *cache_find(int id) { -- cgit v1.1 From ded5e5ed2f3348ba2f9a319c6497e46c22850e97 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 19 May 2010 10:46:55 -0700 Subject: Update call_rcu() usage, add synchronize_rcu() Reported-by: Kyle Hubert Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett --- Documentation/DocBook/kernel-locking.tmpl | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'Documentation') diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl index e6cc574..ed64d22 100644 --- a/Documentation/DocBook/kernel-locking.tmpl +++ b/Documentation/DocBook/kernel-locking.tmpl @@ -1645,7 +1645,9 @@ the amount of locking which needs to be done. all the readers who were traversing the list when we deleted the element are finished. We use call_rcu() to register a callback which will actually destroy the object once - the readers are finished. + all pre-existing readers are finished. Alternatively, + synchronize_rcu() may be used to block until + all pre-existing are finished. But how does Read Copy Update know when the readers are @@ -1714,7 +1716,7 @@ the amount of locking which needs to be done. - object_put(obj); + list_del_rcu(&obj->list); cache_num--; -+ call_rcu(&obj->rcu, cache_delete_rcu, obj); ++ call_rcu(&obj->rcu, cache_delete_rcu); } /* Must be holding cache_lock */ -- cgit v1.1 From 5cc6517abdeccb6690b344a43b5ce8eaee82da3c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 13 Aug 2010 16:34:22 -0700 Subject: rcu: document ways of stalling updates in low-memory situations Signed-off-by: Paul E. McKenney --- Documentation/RCU/checklist.txt | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) (limited to 'Documentation') diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index c7c6788..0c134f8 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -218,13 +218,22 @@ over a rather long period of time, but improvements are always welcome! include: a. Keeping a count of the number of data-structure elements - used by the RCU-protected data structure, including those - waiting for a grace period to elapse. Enforce a limit - on this number, stalling updates as needed to allow - previously deferred frees to complete. - - Alternatively, limit only the number awaiting deferred - free rather than the total number of elements. + used by the RCU-protected data structure, including + those waiting for a grace period to elapse. Enforce a + limit on this number, stalling updates as needed to allow + previously deferred frees to complete. Alternatively, + limit only the number awaiting deferred free rather than + the total number of elements. + + One way to stall the updates is to acquire the update-side + mutex. (Don't try this with a spinlock -- other CPUs + spinning on the lock could prevent the grace period + from ever ending.) Another way to stall the updates + is for the updates to use a wrapper function around + the memory allocator, so that this wrapper function + simulates OOM when there is too much memory awaiting an + RCU grace period. There are of course many other + variations on this theme. b. Limiting update rate. For example, if updates occur only once per hour, then no explicit rate limiting is required, -- cgit v1.1