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/* Efficient small actor read-write lock
(C) 2016-2017 Niall Douglas <http://www.nedproductions.biz/> (12 commits)
File Created: March 2016
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License in the accompanying file
Licence.txt or at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
Distributed under the Boost Software License, Version 1.0.
(See accompanying file Licence.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)
*/
#ifndef LLFIO_SHARED_FS_MUTEX_BYTE_RANGES_HPP
#define LLFIO_SHARED_FS_MUTEX_BYTE_RANGES_HPP
#include "../../file_handle.hpp"
#include "base.hpp"
#ifdef __has_include
#if __has_include("../../quickcpplib/include/algorithm/small_prng.hpp")
#include "../../quickcpplib/include/algorithm/small_prng.hpp"
#else
#include "quickcpplib/include/algorithm/small_prng.hpp"
#endif
#elif __PCPP_ALWAYS_TRUE__
#include "quickcpplib/include/algorithm/small_prng.hpp"
#else
#include "../../quickcpplib/include/algorithm/small_prng.hpp"
#endif
//! \file byte_ranges.hpp Provides algorithm::shared_fs_mutex::byte_ranges
LLFIO_V2_NAMESPACE_BEGIN
namespace algorithm
{
namespace shared_fs_mutex
{
/*! \class byte_ranges
\brief Many entity shared/exclusive file system based lock
This is a simple many entity shared mutex. It works by locking in the same file the byte at the
offset of the entity id. If it fails to lock a byte, it backs out all preceding locks, randomises the order
and tries locking them again until success. Needless to say this algorithm puts a lot of strain on
your byte range locking implementation, some NFS implementations have been known to fail to cope.
\note Most users will want to use `safe_byte_ranges` instead of this class directly.
- Compatible with networked file systems, though be cautious with older NFS.
- Linear complexity to number of concurrent users.
- Exponential complexity to number of entities being concurrently locked, though some OSs
provide linear complexity so long as total concurrent waiting processes is CPU core count or less.
- Does a reasonable job of trying to sleep the thread if any of the entities are locked.
- Sudden process exit with lock held is recovered from.
- Sudden power loss during use is recovered from.
- Safe for multithreaded usage of the same instance.
Caveats:
- When entities being locked is more than one, the algorithm places the contending lock at the
front of the list during the randomisation after lock failure so we can sleep the thread until
it becomes free. However, under heavy churn the thread will generally spin, consuming 100% CPU.
- Byte range locks need to work properly on your system. Misconfiguring NFS or Samba
to cause byte range locks to not work right will produce bad outcomes.
- If your OS doesn't have sane byte range locks (OS X, BSD, older Linuxes) and multiple
objects in your process use the same lock file, misoperation will occur. Use lock_files
or share a single instance of this class per lock file in this case.
- If you are on POSIX and the same process relocks an entity a second time, it will release everything
on the first unlock. On Windows, the first unlock releases the exclusive lock and the second
unlock will release the shared lock.
*/
class byte_ranges : public shared_fs_mutex
{
file_handle _h;
explicit byte_ranges(file_handle &&h)
: _h(std::move(h))
{
}
public:
//! The type of an entity id
using entity_type = shared_fs_mutex::entity_type;
//! The type of a sequence of entities
using entities_type = shared_fs_mutex::entities_type;
//! No copy construction
byte_ranges(const byte_ranges &) = delete;
//! No copy assignment
byte_ranges &operator=(const byte_ranges &) = delete;
~byte_ranges() = default;
//! Move constructor
byte_ranges(byte_ranges &&o) noexcept : _h(std::move(o._h)) {}
//! Move assign
byte_ranges &operator=(byte_ranges &&o) noexcept
{
_h = std::move(o._h);
return *this;
}
//! Initialises a shared filing system mutex using the file at \em lockfile
LLFIO_MAKE_FREE_FUNCTION
static result<byte_ranges> fs_mutex_byte_ranges(const path_handle &base, path_view lockfile) noexcept
{
LLFIO_LOG_FUNCTION_CALL(0);
OUTCOME_TRY(ret, file_handle::file(base, lockfile, file_handle::mode::write, file_handle::creation::if_needed, file_handle::caching::temporary));
return byte_ranges(std::move(ret));
}
//! Return the handle to file being used for this lock
const file_handle &handle() const noexcept { return _h; }
protected:
LLFIO_HEADERS_ONLY_VIRTUAL_SPEC result<void> _lock(entities_guard &out, deadline d, bool spin_not_sleep) noexcept final
{
LLFIO_LOG_FUNCTION_CALL(this);
std::chrono::steady_clock::time_point began_steady;
std::chrono::system_clock::time_point end_utc;
if(d)
{
if((d).steady)
{
began_steady = std::chrono::steady_clock::now();
}
else
{
end_utc = (d).to_time_point();
}
}
// Fire this if an error occurs
auto disableunlock = undoer([&] { out.release(); });
size_t n;
for(;;)
{
auto was_contended = static_cast<size_t>(-1);
{
auto undo = undoer([&] {
// 0 to (n-1) need to be closed
if(n > 0)
{
--n;
// Now 0 to n needs to be closed
for(; n > 0; n--)
{
_h.unlock(out.entities[n].value, 1);
}
_h.unlock(out.entities[0].value, 1);
}
});
for(n = 0; n < out.entities.size(); n++)
{
deadline nd;
// Only for very first entity will we sleep until its lock becomes available
if(n != 0u)
{
nd = deadline(std::chrono::seconds(0));
}
else
{
nd = deadline();
if(d)
{
if((d).steady)
{
std::chrono::nanoseconds ns = std::chrono::duration_cast<std::chrono::nanoseconds>((began_steady + std::chrono::nanoseconds((d).nsecs)) - std::chrono::steady_clock::now());
if(ns.count() < 0)
{
(nd).nsecs = 0;
}
else
{
(nd).nsecs = ns.count();
}
}
else
{
(nd) = (d);
}
}
}
auto outcome = _h.lock(out.entities[n].value, 1, out.entities[n].exclusive != 0u, nd);
if(!outcome)
{
was_contended = n;
goto failed;
}
outcome.value().release();
}
// Everything is locked, exit
undo.dismiss();
disableunlock.dismiss();
return success();
}
failed:
if(d)
{
if((d).steady)
{
if(std::chrono::steady_clock::now() >= (began_steady + std::chrono::nanoseconds((d).nsecs)))
{
return errc::timed_out;
}
}
else
{
if(std::chrono::system_clock::now() >= end_utc)
{
return errc::timed_out;
}
}
}
// Move was_contended to front and randomise rest of out.entities
std::swap(out.entities[was_contended], out.entities[0]);
auto front = out.entities.begin();
++front;
QUICKCPPLIB_NAMESPACE::algorithm::small_prng::random_shuffle(front, out.entities.end());
if(!spin_not_sleep)
{
std::this_thread::yield();
}
}
// return success();
}
public:
LLFIO_HEADERS_ONLY_VIRTUAL_SPEC void unlock(entities_type entities, unsigned long long /*hint*/) noexcept final
{
LLFIO_LOG_FUNCTION_CALL(this);
for(const auto &i : entities)
{
_h.unlock(i.value, 1);
}
}
};
} // namespace shared_fs_mutex
} // namespace algorithm
LLFIO_V2_NAMESPACE_END
#endif
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