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Merged
merged 3 commits into from
Sep 2, 2018
Merged

Reduce number of syscalls in rand #53725

merged 3 commits into from
Sep 2, 2018

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tbu-
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@tbu- tbu- commented Aug 26, 2018 •
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This skips the initial zero-length getrandom call and
directly hands the user buffer to the operating system, saving one
getrandom syscall.

@tbu-
Reduce number of syscalls in rand
09a615c 
In case that it is statically known that the OS doesn't support
`getrandom` (non-Linux) or becomes clear at runtime that `getrandom`
isn't available (`ENOSYS`), the opened fd ("/dev/urandom") isn't closed
after the function, so that future calls can reuse it. This saves
repeated `open`/`close` system calls at the cost of one permanently open
fd.

Additionally, this skips the initial zero-length `getrandom` call and
directly hands the user buffer to the operating system, saving one
`getrandom` syscall.
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r? @Mark-Simulacrum

(rust_highfive has picked a reviewer for you, use r? to override)

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 26, 2018
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r? @alexcrichton

@tbu-
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tbu- commented Aug 27, 2018

Program:

use std::collections::HashMap;

fn main() {
    let mut m = HashMap::new();
    m.insert("abc", "def");
}

Without this PR:

[…]
sigaltstack(NULL, {ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f072df95000
sigaltstack({ss_sp=0x7f072df95000, ss_flags=0, ss_size=8192}, NULL) = 0
getrandom("", 0, GRND_NONBLOCK)         = 0
getrandom("\x26\x53\x69\xb8\x75\xd4\xe3\x9a\x32\x11\x10\x58\xa7\x8a\x47\x4b", 16, GRND_NONBLOCK) = 16
sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=8192}, NULL) = 0
munmap(0x7f072df95000, 8192)            = 0
exit_group(0)                           = ?
+++ exited with 0 +++

With this PR:

[…]
sigaltstack(NULL, {ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa8d3862000
sigaltstack({ss_sp=0x7fa8d3862000, ss_flags=0, ss_size=8192}, NULL) = 0
getrandom("\x05\x08\x66\x91\x2a\xf0\x05\xf9\x44\x34\x7c\x27\xf3\xfe\x06\x4c", 16, GRND_NONBLOCK) = 16
sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=8192}, NULL) = 0
munmap(0x7fa8d3862000, 8192)            = 0
exit_group(0)                           = ?
+++ exited with 0 +++

The initial getrandom("", 0, GRND_NONBLOCK) = 0 gets skipped.

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Thanks for the PR! The changes to getrandom seem good to me, but I'm not super comfortable with effectively leaking a permanent file descriptor. This isn't really a performance sensitive piece of code, right? (It's only used sparingly on each thread I think?) In that sense is it really necessary to get the minor boost from leaving it open rather than closing and reopening?

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tbu- commented Aug 29, 2018

Done.

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@bors: r+

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bors commented Aug 29, 2018

📌 Commit 6d47737 has been approved by alexcrichton

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 29, 2018
pietroalbini added a commit to pietroalbini/rust that referenced this pull request Aug 29, 2018
@pietroalbini
Rollup merge of rust-lang#53725 - tbu-:pr_getrandom_syscalls, r=alexc…
b18ecbd 
…richton

Reduce number of syscalls in `rand`

This skips the initial zero-length `getrandom` call and
directly hands the user buffer to the operating system, saving one
`getrandom` syscall.
@pietroalbini pietroalbini mentioned this pull request Aug 29, 2018
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@bors r-

[00:25:39] error: unused import: `libc`
[00:25:39]   --> libstd/sys/unix/rand.rs:32:9
[00:25:39]    |
[00:25:39] 32 |     use libc;
[00:25:39]    |         ^^^^
[00:25:39]    |
[00:25:39]    = note: `-D unused-imports` implied by `-D warnings`
[00:25:39] 
[00:25:39] error: unused import: `sys::os::errno`
[00:25:39]   --> libstd/sys/unix/rand.rs:33:9
[00:25:39]    |
[00:25:39] 33 |     use sys::os::errno;
[00:25:39]    |         ^^^^^^^^^^^^^^
[00:25:39] 
[00:25:43] error: aborting due to 2 previous errors

@bors bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 29, 2018
@tbu- tbu- force-pushed the pr_getrandom_syscalls branch from 6d47737 to d6d280b Compare August 29, 2018 20:30
goffrie
goffrie reviewed Aug 31, 2018
View reviewed changes
src/libstd/sys/unix/rand.rs
let mut read = 0;
while read < v.len() {
let result = getrandom(&mut v[read..]);
if result == -1 {
let err = errno() as libc::c_int;
if err == libc::EINTR {
continue;
} else if err == libc::ENOSYS {
GETRANDOM_UNAVAILABLE.store(true, Ordering::Relaxed);
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Would this not loop infinitely in the ENOSYS case?

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Oh my...

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@bors: r-

oh dear, good eyes @goffrie!

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@bors: r+

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bors commented Sep 2, 2018

📌 Commit b95c491 has been approved by alexcrichton

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 2, 2018
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bors commented Sep 2, 2018

⌛ Testing commit b95c491 with merge 9395f0a...

bors added a commit that referenced this pull request Sep 2, 2018
@bors
Auto merge of #53725 - tbu-:pr_getrandom_syscalls, r=alexcrichton
9395f0a 
Reduce number of syscalls in `rand`

This skips the initial zero-length `getrandom` call and
directly hands the user buffer to the operating system, saving one
`getrandom` syscall.
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bors commented Sep 2, 2018

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 9395f0a to master...

@bors bors merged commit b95c491 into rust-lang:master Sep 2, 2018
@tbu- tbu- mentioned this pull request Mar 6, 2024
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@tbu- @rust-highfive @Mark-Simulacrum @alexcrichton @bors @pietroalbini @goffrie