~ [ source navigation ] ~ [ diff markup ] ~ [ identifier search ] ~

TOMOYO Linux Cross Reference
Linux/kernel/locking/percpu-rwsem.c

Version: ~ [ linux-5.16-rc3 ] ~ [ linux-5.15.5 ] ~ [ linux-5.14.21 ] ~ [ linux-5.13.19 ] ~ [ linux-5.12.19 ] ~ [ linux-5.11.22 ] ~ [ linux-5.10.82 ] ~ [ linux-5.9.16 ] ~ [ linux-5.8.18 ] ~ [ linux-5.7.19 ] ~ [ linux-5.6.19 ] ~ [ linux-5.5.19 ] ~ [ linux-5.4.162 ] ~ [ linux-5.3.18 ] ~ [ linux-5.2.21 ] ~ [ linux-5.1.21 ] ~ [ linux-5.0.21 ] ~ [ linux-4.20.17 ] ~ [ linux-4.19.218 ] ~ [ linux-4.18.20 ] ~ [ linux-4.17.19 ] ~ [ linux-4.16.18 ] ~ [ linux-4.15.18 ] ~ [ linux-4.14.256 ] ~ [ linux-4.13.16 ] ~ [ linux-4.12.14 ] ~ [ linux-4.11.12 ] ~ [ linux-4.10.17 ] ~ [ linux-4.9.291 ] ~ [ linux-4.8.17 ] ~ [ linux-4.7.10 ] ~ [ linux-4.6.7 ] ~ [ linux-4.5.7 ] ~ [ linux-4.4.293 ] ~ [ linux-4.3.6 ] ~ [ linux-4.2.8 ] ~ [ linux-4.1.52 ] ~ [ linux-4.0.9 ] ~ [ linux-3.18.140 ] ~ [ linux-3.16.85 ] ~ [ linux-3.14.79 ] ~ [ linux-3.12.74 ] ~ [ linux-3.10.108 ] ~ [ linux-2.6.32.71 ] ~ [ linux-2.6.0 ] ~ [ linux-2.4.37.11 ] ~ [ unix-v6-master ] ~ [ ccs-tools-1.8.5 ] ~ [ policy-sample ] ~
Architecture: ~ [ i386 ] ~ [ alpha ] ~ [ m68k ] ~ [ mips ] ~ [ ppc ] ~ [ sparc ] ~ [ sparc64 ] ~

  1 #include <linux/atomic.h>
  2 #include <linux/rwsem.h>
  3 #include <linux/percpu.h>
  4 #include <linux/wait.h>
  5 #include <linux/lockdep.h>
  6 #include <linux/percpu-rwsem.h>
  7 #include <linux/rcupdate.h>
  8 #include <linux/sched.h>
  9 #include <linux/errno.h>
 10 
 11 int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
 12                         const char *name, struct lock_class_key *rwsem_key)
 13 {
 14         brw->fast_read_ctr = alloc_percpu(int);
 15         if (unlikely(!brw->fast_read_ctr))
 16                 return -ENOMEM;
 17 
 18         /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
 19         __init_rwsem(&brw->rw_sem, name, rwsem_key);
 20         atomic_set(&brw->write_ctr, 0);
 21         atomic_set(&brw->slow_read_ctr, 0);
 22         init_waitqueue_head(&brw->write_waitq);
 23         return 0;
 24 }
 25 
 26 void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
 27 {
 28         free_percpu(brw->fast_read_ctr);
 29         brw->fast_read_ctr = NULL; /* catch use after free bugs */
 30 }
 31 
 32 /*
 33  * This is the fast-path for down_read/up_read, it only needs to ensure
 34  * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
 35  * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
 36  * serialize with the preempt-disabled section below.
 37  *
 38  * The nontrivial part is that we should guarantee acquire/release semantics
 39  * in case when
 40  *
 41  *      R_W: down_write() comes after up_read(), the writer should see all
 42  *           changes done by the reader
 43  * or
 44  *      W_R: down_read() comes after up_write(), the reader should see all
 45  *           changes done by the writer
 46  *
 47  * If this helper fails the callers rely on the normal rw_semaphore and
 48  * atomic_dec_and_test(), so in this case we have the necessary barriers.
 49  *
 50  * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
 51  * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
 52  * reader inside the critical section. See the comments in down_write and
 53  * up_write below.
 54  */
 55 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 56 {
 57         bool success = false;
 58 
 59         preempt_disable();
 60         if (likely(!atomic_read(&brw->write_ctr))) {
 61                 __this_cpu_add(*brw->fast_read_ctr, val);
 62                 success = true;
 63         }
 64         preempt_enable();
 65 
 66         return success;
 67 }
 68 
 69 /*
 70  * Like the normal down_read() this is not recursive, the writer can
 71  * come after the first percpu_down_read() and create the deadlock.
 72  *
 73  * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
 74  * percpu_up_read() does rwsem_release(). This pairs with the usage
 75  * of ->rw_sem in percpu_down/up_write().
 76  */
 77 void percpu_down_read(struct percpu_rw_semaphore *brw)
 78 {
 79         might_sleep();
 80         if (likely(update_fast_ctr(brw, +1))) {
 81                 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
 82                 return;
 83         }
 84 
 85         down_read(&brw->rw_sem);
 86         atomic_inc(&brw->slow_read_ctr);
 87         /* avoid up_read()->rwsem_release() */
 88         __up_read(&brw->rw_sem);
 89 }
 90 
 91 void percpu_up_read(struct percpu_rw_semaphore *brw)
 92 {
 93         rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
 94 
 95         if (likely(update_fast_ctr(brw, -1)))
 96                 return;
 97 
 98         /* false-positive is possible but harmless */
 99         if (atomic_dec_and_test(&brw->slow_read_ctr))
100                 wake_up_all(&brw->write_waitq);
101 }
102 
103 static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
104 {
105         unsigned int sum = 0;
106         int cpu;
107 
108         for_each_possible_cpu(cpu) {
109                 sum += per_cpu(*brw->fast_read_ctr, cpu);
110                 per_cpu(*brw->fast_read_ctr, cpu) = 0;
111         }
112 
113         return sum;
114 }
115 
116 /*
117  * A writer increments ->write_ctr to force the readers to switch to the
118  * slow mode, note the atomic_read() check in update_fast_ctr().
119  *
120  * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
121  * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
122  * counter it represents the number of active readers.
123  *
124  * Finally the writer takes ->rw_sem for writing and blocks the new readers,
125  * then waits until the slow counter becomes zero.
126  */
127 void percpu_down_write(struct percpu_rw_semaphore *brw)
128 {
129         /* tell update_fast_ctr() there is a pending writer */
130         atomic_inc(&brw->write_ctr);
131         /*
132          * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
133          *    so that update_fast_ctr() can't succeed.
134          *
135          * 2. Ensures we see the result of every previous this_cpu_add() in
136          *    update_fast_ctr().
137          *
138          * 3. Ensures that if any reader has exited its critical section via
139          *    fast-path, it executes a full memory barrier before we return.
140          *    See R_W case in the comment above update_fast_ctr().
141          */
142         synchronize_sched_expedited();
143 
144         /* exclude other writers, and block the new readers completely */
145         down_write(&brw->rw_sem);
146 
147         /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
148         atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
149 
150         /* wait for all readers to complete their percpu_up_read() */
151         wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
152 }
153 
154 void percpu_up_write(struct percpu_rw_semaphore *brw)
155 {
156         /* release the lock, but the readers can't use the fast-path */
157         up_write(&brw->rw_sem);
158         /*
159          * Insert the barrier before the next fast-path in down_read,
160          * see W_R case in the comment above update_fast_ctr().
161          */
162         synchronize_sched_expedited();
163         /* the last writer unblocks update_fast_ctr() */
164         atomic_dec(&brw->write_ctr);
165 }
166 

~ [ source navigation ] ~ [ diff markup ] ~ [ identifier search ] ~

kernel.org | git.kernel.org | LWN.net | Project Home | Wiki (Japanese) | Wiki (English) | SVN repository | Mail admin

Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.

osdn.jp