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Subject[PATCH 04/32] locking/lockdep: Test all incompatible scenario at once in check_irq_usage()
Date
check_prev_add_irq() tests all incompatible scenarios one after the
other while adding a lock (@next) to a tree dependency (@prev):

LOCK_USED_IN_HARDIRQ vs LOCK_ENABLED_HARDIRQ
LOCK_USED_IN_HARDIRQ_READ vs LOCK_ENABLED_HARDIRQ
LOCK_USED_IN_SOFTIRQ vs LOCK_ENABLED_SOFTIRQ
LOCK_USED_IN_SOFTIRQ_READ vs LOCK_ENABLED_SOFTIRQ

Also for these four scenarios, we must at least iterate the @prev
backward dependency. Then if it matches the relevant LOCK_USED_* bit,
we must also iterate the @next forward dependency.

Therefore in the best case we iterate 4 times, in the worst case 8 times.

Now things are going to become much worse as we plan to add as much
LOCK_USED_IN_{$VEC}_SOFTIRQ[_READ] as we have softirq vectors, currently
10. So the expanded test would be at least 22 loops and at most 44. We
can't really afford that.

So we take a different approach to fix this:

1) Iterate through @prev backward dependencies and accumulate all the IRQ
uses in a single mask.

2) Iterate through @next forward dependencies and try to find a lock
whose usage is exclusive to the accumulated usages gathered in the
previous step. If we find one (call it @lockA), we have found an
incompatible use.

3) Iterate again through @prev backward dependency and find the lock
whose usage matches @lockA in term of incompatibility. Call that
lock @lockB.

4) Report the incompatible usages of @lockA and @lockB

If no incompatible use is found, the verification never goes beyond
step 2 which means at most two iterations.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Pavan Kondeti <pkondeti@codeaurora.org>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
kernel/locking/lockdep.c | 204 ++++++++++++++++++++++++++-------------
1 file changed, 137 insertions(+), 67 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a977aa5976b7..578dd57f70d3 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1329,6 +1329,14 @@ check_redundant(struct lock_list *root, struct lock_class *target,
}

#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
+
+static inline int usage_accumulate(struct lock_list *entry, void *mask)
+{
+ *(u64 *)mask |= entry->class->usage_mask;
+
+ return 0;
+}
+
/*
* Forwards and backwards subgraph searching, for the purposes of
* proving that two subgraphs can be connected by a new dependency
@@ -1340,8 +1348,6 @@ static inline int usage_match(struct lock_list *entry, void *mask)
return entry->class->usage_mask & *(u64 *)mask;
}

-
-
/*
* Find a node in the forwards-direction dependency sub-graph starting
* at @root->class that matches @bit.
@@ -1575,39 +1581,6 @@ print_bad_irq_dependency(struct task_struct *curr,
return 0;
}

-static int
-check_usage(struct task_struct *curr, struct held_lock *prev,
- struct held_lock *next, enum lock_usage_bit bit_backwards,
- enum lock_usage_bit bit_forwards, const char *irqclass)
-{
- int ret;
- struct lock_list this, that;
- struct lock_list *uninitialized_var(target_entry);
- struct lock_list *uninitialized_var(target_entry1);
-
- this.parent = NULL;
-
- this.class = hlock_class(prev);
- ret = find_usage_backwards(&this, BIT(bit_backwards), &target_entry);
- if (ret < 0)
- return print_bfs_bug(ret);
- if (ret == 1)
- return ret;
-
- that.parent = NULL;
- that.class = hlock_class(next);
- ret = find_usage_forwards(&that, BIT(bit_forwards), &target_entry1);
- if (ret < 0)
- return print_bfs_bug(ret);
- if (ret == 1)
- return ret;
-
- return print_bad_irq_dependency(curr, &this, &that,
- target_entry, target_entry1,
- prev, next,
- bit_backwards, bit_forwards, irqclass);
-}
-
static const char *state_names[] = {
#define LOCKDEP_STATE(__STATE) \
__stringify(__STATE),
@@ -1638,45 +1611,143 @@ static int exclusive_bit(int new_bit)
return state | (dir ^ LOCK_USAGE_DIR_MASK);
}

+static u64 __invert_mask(u64 mask, bool read)
+{
+ u64 excl_mask = 0;
+ int bit = 0;
+
+ while (mask) {
+ long fs = __ffs64(mask) + 1;
+ int excl;
+
+ mask >>= fs;
+ bit += fs;
+ excl = exclusive_bit(bit - 1);
+ excl_mask |= lock_flag(excl);
+ if (read)
+ excl_mask |= lock_flag(excl | LOCK_USAGE_READ_MASK);
+ }
+
+ return excl_mask;
+}
+
+static u64 exclusive_mask(u64 mask)
+{
+ return __invert_mask(mask, false);
+}
+
+/*
+ * Retrieve the _possible_ original mask to which @mask is
+ * exclusive. Ie: this is the opposite of exclusive_mask().
+ * Note that 2 possible original bits can match an exclusive
+ * bit: one has LOCK_USAGE_READ_MASK set, the other has it
+ * cleared. So both are returned for each exclusive bit.
+ */
+static u64 original_mask(u64 mask)
+{
+ return __invert_mask(mask, true);
+}
+
+/*
+ * Find the first pair of bit match between an original
+ * usage mask and an exclusive usage mask.
+ */
+static int find_exclusive_match(u64 mask, u64 excl_mask,
+ enum lock_usage_bit *bit,
+ enum lock_usage_bit *excl_bit)
+{
+ int nr = 0;
+
+ while (mask) {
+ long fs = __ffs64(mask) + 1;
+ int excl;
+
+ mask >>= fs;
+ nr += fs;
+
+ excl = exclusive_bit(nr - 1);
+ if (excl_mask & lock_flag(excl)) {
+ *bit = nr - 1;
+ *excl_bit = excl;
+ return 0;
+ }
+ }
+ return -1;
+}
+
+/*
+ * Prove that the new dependency does not connect a hardirq-safe(-read)
+ * lock with a hardirq-unsafe lock - to achieve this we search
+ * the backwards-subgraph starting at <prev>, and the
+ * forwards-subgraph starting at <next>:
+ */
static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
- struct held_lock *next, enum lock_usage_bit bit)
+ struct held_lock *next)
{
+ enum lock_usage_bit forward_bit = 0, backward_bit = 0;
+ struct lock_list *uninitialized_var(target_entry1);
+ struct lock_list *uninitialized_var(target_entry);
+ u64 usage_mask = 0, forward_mask, backward_mask;
+ struct lock_list this, that;
+ int ret;
+
/*
- * Prove that the new dependency does not connect a hardirq-safe
- * lock with a hardirq-unsafe lock - to achieve this we search
- * the backwards-subgraph starting at <prev>, and the
- * forwards-subgraph starting at <next>:
+ * Step 1: gather all hard/soft IRQs usages backward in an
+ * accumulated usage mask.
*/
- if (!check_usage(curr, prev, next, bit,
- exclusive_bit(bit), state_name(bit)))
- return 0;
+ this.parent = NULL;
+ this.class = hlock_class(prev);
+
+ ret = __bfs_backwards(&this, &usage_mask, usage_accumulate, NULL);
+ if (ret < 0)
+ return print_bfs_bug(ret);

- bit++; /* _READ */
+ usage_mask &= LOCKF_USED_IN_IRQ | LOCKF_USED_IN_IRQ_READ;
+ if (!usage_mask)
+ return 1;

/*
- * Prove that the new dependency does not connect a hardirq-safe-read
- * lock with a hardirq-unsafe lock - to achieve this we search
- * the backwards-subgraph starting at <prev>, and the
- * forwards-subgraph starting at <next>:
+ * Step 2: find exclusive uses forward that match the previous
+ * backward accumulated mask.
*/
- if (!check_usage(curr, prev, next, bit,
- exclusive_bit(bit), state_name(bit)))
- return 0;
+ forward_mask = exclusive_mask(usage_mask);

- return 1;
-}
+ that.parent = NULL;
+ that.class = hlock_class(next);

-static int
-check_prev_add_irq(struct task_struct *curr, struct held_lock *prev,
- struct held_lock *next)
-{
-#define LOCKDEP_STATE(__STATE) \
- if (!check_irq_usage(curr, prev, next, LOCK_USED_IN_##__STATE)) \
- return 0;
-#include "lockdep_states.h"
-#undef LOCKDEP_STATE
+ ret = find_usage_forwards(&that, forward_mask, &target_entry1);
+ if (ret < 0)
+ return print_bfs_bug(ret);
+ if (ret == 1)
+ return ret;
+
+ /*
+ * Step 3: we found a bad match! Now retrieve a lock from the backward
+ * list whose usage mask matches the exclusive usage mask from the
+ * lock found on the forward list.
+ */
+ backward_mask = original_mask(target_entry1->class->usage_mask);
+
+ ret = find_usage_backwards(&this, backward_mask, &target_entry);
+ if (ret < 0)
+ return print_bfs_bug(ret);
+ if (ret == 1)
+ return print_bfs_bug(ret);
+
+ /*
+ * Step 4: narrow down to a pair of incompatible usage bits
+ * and report it.
+ */
+ ret = find_exclusive_match(target_entry->class->usage_mask,
+ target_entry1->class->usage_mask,
+ &backward_bit, &forward_bit);
+ WARN_ON_ONCE(ret == -1);

- return 1;
+ return print_bad_irq_dependency(curr, &this, &that,
+ target_entry, target_entry1,
+ prev, next,
+ backward_bit, forward_bit,
+ state_name(backward_bit));
}

static void inc_chains(void)
@@ -1693,9 +1764,8 @@ static void inc_chains(void)

#else

-static inline int
-check_prev_add_irq(struct task_struct *curr, struct held_lock *prev,
- struct held_lock *next)
+static inline int check_irq_usage(struct task_struct *curr,
+ struct held_lock *prev, struct held_lock *next)
{
return 1;
}
@@ -1857,7 +1927,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
else if (unlikely(ret < 0))
return print_bfs_bug(ret);

- if (!check_prev_add_irq(curr, prev, next))
+ if (!check_irq_usage(curr, prev, next))
return 0;

/*
--
2.17.1
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