proc: do less stuff under ->pde_unload_lock

Commit ca469f35a8e9ef ("deal with races between remove_proc_entry() and
proc_reg_release()") moved too much stuff under ->pde_unload_lock making
a problem described at series "[PATCH v5] procfs: Improve Scaling in
proc" worse.

While RCU is being figured out, move kfree() out of ->pde_unload_lock.

On my potato, difference is only 0.5% speedup with concurrent
open+read+close of /proc/cmdline, but the effect should be more
noticeable on more capable machines.

$ perf stat -r 16 -- ./proc-j 16

 Performance counter stats for './proc-j 16' (16 runs):

     130569.502377      task-clock (msec)         #   15.872 CPUs utilized            ( +-  0.05% )
            19,169      context-switches          #    0.147 K/sec                    ( +-  0.18% )
                15      cpu-migrations            #    0.000 K/sec                    ( +-  3.27% )
               437      page-faults               #    0.003 K/sec                    ( +-  1.25% )
   300,172,097,675      cycles                    #    2.299 GHz                      ( +-  0.05% )
    96,793,267,308      instructions              #    0.32  insn per cycle           ( +-  0.04% )
    22,798,342,298      branches                  #  174.607 M/sec                    ( +-  0.04% )
       111,764,687      branch-misses             #    0.49% of all branches          ( +-  0.47% )

       8.226574400 seconds time elapsed                                          ( +-  0.05% )
       ^^^^^^^^^^^

$ perf stat -r 16 -- ./proc-j 16

 Performance counter stats for './proc-j 16' (16 runs):

     129866.777392      task-clock (msec)         #   15.869 CPUs utilized            ( +-  0.04% )
            19,154      context-switches          #    0.147 K/sec                    ( +-  0.66% )
                14      cpu-migrations            #    0.000 K/sec                    ( +-  1.73% )
               431      page-faults               #    0.003 K/sec                    ( +-  1.09% )
   298,556,520,546      cycles                    #    2.299 GHz                      ( +-  0.04% )
    96,525,366,833      instructions              #    0.32  insn per cycle           ( +-  0.04% )
    22,730,194,043      branches                  #  175.027 M/sec                    ( +-  0.04% )
       111,506,074      branch-misses             #    0.49% of all branches          ( +-  0.18% )

       8.183629778 seconds time elapsed                                          ( +-  0.04% )
       ^^^^^^^^^^^

Link: http://lkml.kernel.org/r/20180213132911.GA24298@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 6e8724958116..8118ce5df5c6 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -138,7 +138,7 @@ static void unuse_pde(struct proc_dir_entry *pde)
                 complete(pde->pde_unload_completion);
 }
 
-/* pde is locked */
+/* pde is locked on entry, unlocked on exit */
 static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 {
         /*
@@ -157,9 +157,10 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
                 pdeo->c = &c;
                 spin_unlock(&pde->pde_unload_lock);
                 wait_for_completion(&c);
-                spin_lock(&pde->pde_unload_lock);
         } else {
                 struct file *file;
+                struct completion *c;
+
                 pdeo->closing = true;
                 spin_unlock(&pde->pde_unload_lock);
                 file = pdeo->file;
@@ -167,8 +168,10 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
                 spin_lock(&pde->pde_unload_lock);
                 /* After ->release. */
                 list_del(&pdeo->lh);
-                if (unlikely(pdeo->c))
-                        complete(pdeo->c);
+                c = pdeo->c;
+                spin_unlock(&pde->pde_unload_lock);
+                if (unlikely(c))
+                        complete(c);
                 kfree(pdeo);
         }
 }
@@ -188,6 +191,7 @@ void proc_entry_rundown(struct proc_dir_entry *de)
                 struct pde_opener *pdeo;
                 pdeo = list_first_entry(&de->pde_openers, struct pde_opener, lh);
                 close_pdeo(de, pdeo);
+                spin_lock(&de->pde_unload_lock);
         }
         spin_unlock(&de->pde_unload_lock);
 }
@@ -375,7 +379,7 @@ static int proc_reg_release(struct inode *inode, struct file *file)
         list_for_each_entry(pdeo, &pde->pde_openers, lh) {
                 if (pdeo->file == file) {
                         close_pdeo(pde, pdeo);
-                        break;
+                        return 0;
                 }
         }
         spin_unlock(&pde->pde_unload_lock);